diff --git a/pkg/vm/stack.go b/pkg/vm/stack.go
index fd1bd275f..a08882d8d 100644
--- a/pkg/vm/stack.go
+++ b/pkg/vm/stack.go
@@ -132,21 +132,15 @@ func (s *Stack) insert(e, at *Element) *Element {
 	return e
 }
 
-// InsertBefore will insert the element before the mark on the stack.
-func (s *Stack) InsertBefore(e, mark *Element) *Element {
-	if mark == nil {
-		return nil
-	}
-	return s.insert(e, mark.prev)
-}
-
 // InsertAt will insert the given item (n) deep on the stack.
+// Be very careful using it and _always_ check both e and n before invocation
+// as it will silently do wrong things otherwise.
 func (s *Stack) InsertAt(e *Element, n int) *Element {
-	before := s.Peek(n)
+	before := s.Peek(n - 1)
 	if before == nil {
 		return nil
 	}
-	return s.InsertBefore(e, before)
+	return s.insert(e, before)
 }
 
 // Push pushes the given element on the stack.
diff --git a/pkg/vm/vm.go b/pkg/vm/vm.go
index a7d9e4c3b..422e8e9e6 100644
--- a/pkg/vm/vm.go
+++ b/pkg/vm/vm.go
@@ -290,6 +290,26 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		v.estack.Push(a)
 		v.estack.Push(b)
 
+	case TUCK:
+		a := v.estack.Dup(0)
+		if a == nil {
+			panic("no top-level element found")
+		}
+		if v.estack.Len() < 2 {
+			panic("can't TUCK with a one-element stack")
+		}
+		v.estack.InsertAt(a, 2)
+
+	case XDROP:
+		n := int(v.estack.Pop().BigInt().Int64())
+		if n < 0 {
+			panic("invalid length")
+		}
+		e := v.estack.RemoveAt(n)
+		if e == nil {
+			panic("bad index")
+		}
+
 	case XSWAP:
 		n := int(v.estack.Pop().BigInt().Int64())
 		if n < 0 {
@@ -306,13 +326,19 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			b.value = aval
 		}
 
-	case TUCK:
+	case XTUCK:
 		n := int(v.estack.Pop().BigInt().Int64())
-		if n <= 0 {
-			panic("OTUCK: invalid length")
+		if n < 0 {
+			panic("XTUCK: invalid length")
 		}
-
-		v.estack.InsertAt(v.estack.Peek(0), n)
+		a := v.estack.Dup(0)
+		if a == nil {
+			panic("no top-level element found")
+		}
+		if n > v.estack.Len() {
+			panic("can't push to the position specified")
+		}
+		v.estack.InsertAt(a, n)
 
 	case ROT:
 		c := v.estack.Pop()
@@ -333,10 +359,27 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 
 	case OVER:
 		b := v.estack.Pop()
+		if b == nil {
+			panic("no top-level element found")
+		}
 		a := v.estack.Peek(0)
+		if a == nil {
+			panic("no second element found")
+		}
 		v.estack.Push(b)
 		v.estack.Push(a)
 
+	case PICK:
+		n := int(v.estack.Pop().BigInt().Int64())
+		if n < 0 {
+			panic("negative stack item returned")
+		}
+		a := v.estack.Peek(n)
+		if a == nil {
+			panic("no nth element found")
+		}
+		v.estack.Push(a)
+
 	case ROLL:
 		n := int(v.estack.Pop().BigInt().Int64())
 		if n < 0 {
@@ -359,6 +402,11 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		v.estack.PushVal(reflect.DeepEqual(a,b))
 
 	// Bit operations.
+	case INVERT:
+		// inplace
+		a := v.estack.Peek(0).BigInt()
+		a.Not(a)
+
 	case AND:
 		b := v.estack.Pop().BigInt()
 		a := v.estack.Pop().BigInt()
@@ -499,8 +547,8 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		v.estack.PushVal(!x)
 
 	case NZ:
-		panic("todo NZ")
-		// x := v.estack.Pop().BigInt()
+		x := v.estack.Pop().BigInt()
+		v.estack.PushVal(x.Cmp(big.NewInt(0)) != 0)
 
 	// Object operations.
 	case NEWARRAY:
@@ -529,10 +577,6 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			panic("APPEND: not of underlying type Array")
 		}
 
-	case REVERSE:
-
-	case REMOVE:
-
 	case PACK:
 		n := int(v.estack.Pop().BigInt().Int64())
 		if n < 0 || n > v.estack.Len() {
@@ -546,9 +590,6 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 
 		v.estack.PushVal(items)
 
-	case UNPACK:
-		panic("TODO")
-
 	case PICKITEM:
 		var (
 			key   = v.estack.Pop()
@@ -671,6 +712,10 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 			v.state = haltState
 		}
 
+	case CAT, SUBSTR, LEFT, RIGHT, CHECKSIG, CHECKMULTISIG,
+		UNPACK, REVERSE, REMOVE:
+		panic("unimplemented")
+
 	// Cryptographic operations.
 	case SHA1:
 		b := v.estack.Pop().Bytes()
@@ -690,12 +735,6 @@ func (v *VM) execute(ctx *Context, op Instruction) {
 		b := v.estack.Pop().Bytes()
 		v.estack.PushVal(hash.DoubleSha256(b).Bytes())
 
-	case CHECKSIG:
-		// pubkey := v.estack.Pop().Bytes()
-		// sig := v.estack.Pop().Bytes()
-
-	case CHECKMULTISIG:
-
 	case NOP:
 		// unlucky ^^
 
diff --git a/pkg/vm/vm_test.go b/pkg/vm/vm_test.go
index 257271056..7abb7bc72 100644
--- a/pkg/vm/vm_test.go
+++ b/pkg/vm/vm_test.go
@@ -23,6 +23,7 @@ func TestInteropHook(t *testing.T) {
 	EmitOpcode(buf, RET)
 	v.Load(buf.Bytes())
 	v.Run()
+	assert.Equal(t, false, v.state.HasFlag(faultState))
 	assert.Equal(t, 1, v.estack.Len())
 	assert.Equal(t, big.NewInt(1), v.estack.Pop().value.Value())
 }
@@ -100,6 +101,7 @@ func TestAdd(t *testing.T) {
 	vm.estack.PushVal(4)
 	vm.estack.PushVal(2)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, int64(6), vm.estack.Pop().BigInt().Int64())
 }
 
@@ -109,6 +111,7 @@ func TestMul(t *testing.T) {
 	vm.estack.PushVal(4)
 	vm.estack.PushVal(2)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, int64(8), vm.estack.Pop().BigInt().Int64())
 }
 
@@ -118,6 +121,7 @@ func TestDiv(t *testing.T) {
 	vm.estack.PushVal(4)
 	vm.estack.PushVal(2)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, int64(2), vm.estack.Pop().BigInt().Int64())
 }
 
@@ -127,6 +131,7 @@ func TestSub(t *testing.T) {
 	vm.estack.PushVal(4)
 	vm.estack.PushVal(2)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, int64(2), vm.estack.Pop().BigInt().Int64())
 }
 
@@ -136,6 +141,7 @@ func TestLT(t *testing.T) {
 	vm.estack.PushVal(4)
 	vm.estack.PushVal(3)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, false, vm.estack.Pop().Bool())
 }
 
@@ -145,6 +151,7 @@ func TestLTE(t *testing.T) {
 	vm.estack.PushVal(2)
 	vm.estack.PushVal(3)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, true, vm.estack.Pop().Bool())
 }
 
@@ -154,6 +161,7 @@ func TestGT(t *testing.T) {
 	vm.estack.PushVal(9)
 	vm.estack.PushVal(3)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, true, vm.estack.Pop().Bool())
 
 }
@@ -164,6 +172,7 @@ func TestGTE(t *testing.T) {
 	vm.estack.PushVal(3)
 	vm.estack.PushVal(3)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, true, vm.estack.Pop().Bool())
 }
 
@@ -174,6 +183,7 @@ func TestDepth(t *testing.T) {
 	vm.estack.PushVal(2)
 	vm.estack.PushVal(3)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, int64(3), vm.estack.Pop().BigInt().Int64())
 }
 
@@ -183,6 +193,7 @@ func TestNumEqual(t *testing.T) {
 	vm.estack.PushVal(1)
 	vm.estack.PushVal(2)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, false, vm.estack.Pop().Bool())
 }
 
@@ -192,6 +203,7 @@ func TestNumNotEqual(t *testing.T) {
 	vm.estack.PushVal(2)
 	vm.estack.PushVal(2)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, false, vm.estack.Pop().Bool())
 }
 
@@ -200,6 +212,7 @@ func TestINC(t *testing.T) {
 	vm := load(prog)
 	vm.estack.PushVal(1)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, big.NewInt(2), vm.estack.Pop().BigInt())
 }
 
@@ -214,6 +227,7 @@ func TestAppCall(t *testing.T) {
 	vm.estack.PushVal(2)
 
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	elem := vm.estack.Pop() // depth should be 1
 	assert.Equal(t, int64(1), elem.BigInt().Int64())
 }
@@ -228,9 +242,245 @@ func TestSimpleCall(t *testing.T) {
 	}
 	vm := load(prog)
 	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
 	assert.Equal(t, result, int(vm.estack.Pop().BigInt().Int64()))
 }
 
+func TestNZtrue(t *testing.T) {
+	prog := makeProgram(NZ)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, true, vm.estack.Pop().Bool())
+}
+
+func TestNZfalse(t *testing.T) {
+	prog := makeProgram(NZ)
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, false, vm.estack.Pop().Bool())
+}
+
+func TestPICKbadNoitem(t *testing.T) {
+	prog := makeProgram(PICK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestPICKbadNegative(t *testing.T) {
+	prog := makeProgram(PICK)
+	vm := load(prog)
+	vm.estack.PushVal(-1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestPICKgood(t *testing.T) {
+	prog := makeProgram(PICK)
+	result := 2
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(result)
+	vm.estack.PushVal(3)
+	vm.estack.PushVal(4)
+	vm.estack.PushVal(5)
+	vm.estack.PushVal(3)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(result), vm.estack.Pop().BigInt().Int64())
+}
+
+func TestXTUCKbadNoitem(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXTUCKbadNoN(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXTUCKbadNegative(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	vm := load(prog)
+	vm.estack.PushVal(-1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXTUCKgood(t *testing.T) {
+	prog := makeProgram(XTUCK)
+	topelement := 5
+	xtuckdepth := 3
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(3)
+	vm.estack.PushVal(4)
+	vm.estack.PushVal(topelement)
+	vm.estack.PushVal(xtuckdepth)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(topelement), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(topelement), vm.estack.Peek(xtuckdepth).BigInt().Int64())
+}
+
+func TestTUCKbadNoitems(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestTUCKbadNoitem(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestTUCKgood(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.estack.PushVal(42)
+	vm.estack.PushVal(34)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(34), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(42), vm.estack.Peek(1).BigInt().Int64())
+	assert.Equal(t, int64(34), vm.estack.Peek(2).BigInt().Int64())
+}
+
+func TestTUCKgood2(t *testing.T) {
+	prog := makeProgram(TUCK)
+	vm := load(prog)
+	vm.estack.PushVal(11)
+	vm.estack.PushVal(42)
+	vm.estack.PushVal(34)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(34), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(42), vm.estack.Peek(1).BigInt().Int64())
+	assert.Equal(t, int64(34), vm.estack.Peek(2).BigInt().Int64())
+	assert.Equal(t, int64(11), vm.estack.Peek(3).BigInt().Int64())
+}
+
+func TestOVERbadNoitem(t *testing.T) {
+	prog := makeProgram(OVER)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestOVERbadNoitems(t *testing.T) {
+	prog := makeProgram(OVER)
+	vm := load(prog)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestOVERgood(t *testing.T) {
+	prog := makeProgram(OVER)
+	vm := load(prog)
+	vm.estack.PushVal(42)
+	vm.estack.PushVal(34)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(42), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(34), vm.estack.Peek(1).BigInt().Int64())
+	assert.Equal(t, int64(42), vm.estack.Peek(2).BigInt().Int64())
+	assert.Equal(t, 3, vm.estack.Len())
+}
+
+func TestXDROPbadNoitem(t *testing.T) {
+	prog := makeProgram(XDROP)
+	vm := load(prog)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXDROPbadNoN(t *testing.T) {
+	prog := makeProgram(XDROP)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXDROPbadNegative(t *testing.T) {
+	prog := makeProgram(XDROP)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(-1)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestXDROPgood(t *testing.T) {
+	prog := makeProgram(XDROP)
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(2)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, 2, vm.estack.Len())
+	assert.Equal(t, int64(2), vm.estack.Peek(0).BigInt().Int64())
+	assert.Equal(t, int64(1), vm.estack.Peek(1).BigInt().Int64())
+}
+
+func TestINVERTbadNoitem(t *testing.T) {
+	prog := makeProgram(INVERT)
+	vm := load(prog)
+	vm.Run()
+	assert.Equal(t, true, vm.state.HasFlag(faultState))
+}
+
+func TestINVERTgood1(t *testing.T) {
+	prog := makeProgram(INVERT)
+	vm := load(prog)
+	vm.estack.PushVal(0)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(-1), vm.estack.Peek(0).BigInt().Int64())
+}
+
+func TestINVERTgood2(t *testing.T) {
+	prog := makeProgram(INVERT)
+	vm := load(prog)
+	vm.estack.PushVal(-1)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(0), vm.estack.Peek(0).BigInt().Int64())
+}
+
+func TestINVERTgood3(t *testing.T) {
+	prog := makeProgram(INVERT)
+	vm := load(prog)
+	vm.estack.PushVal(0x69)
+	vm.Run()
+	assert.Equal(t, false, vm.state.HasFlag(faultState))
+	assert.Equal(t, int64(-0x6A), vm.estack.Peek(0).BigInt().Int64())
+}
+
 func makeProgram(opcodes ...Instruction) []byte {
 	prog := make([]byte, len(opcodes)+1) // RET
 	for i := 0; i < len(opcodes); i++ {