diff --git a/vm/native.go b/vm/native.go
index b7339bc3..003f557f 100644
--- a/vm/native.go
+++ b/vm/native.go
@@ -52,6 +52,7 @@ func sha256Func(input []byte, gas *int64) (output []byte, err error) {
 	}
 	// Hash
 	hasher := sha256.New()
+	// CONTRACT: this does not err
 	_, err = hasher.Write(input)
 	if err != nil {
 		panic(err)
@@ -69,6 +70,7 @@ func ripemd160Func(input []byte, gas *int64) (output []byte, err error) {
 	}
 	// Hash
 	hasher := ripemd160.New()
+	// CONTRACT: this does not err
 	_, err = hasher.Write(input)
 	if err != nil {
 		panic(err)
diff --git a/vm/stack.go b/vm/stack.go
index 12bc1b77..4e7278f9 100644
--- a/vm/stack.go
+++ b/vm/stack.go
@@ -47,6 +47,7 @@ func (st *Stack) Push(d Word256) {
 	st.ptr++
 }
 
+// currently only called after Sha3
 func (st *Stack) PushBytes(bz []byte) {
 	if len(bz) != 32 {
 		panic("Invalid bytes size: expected 32")
diff --git a/vm/vm.go b/vm/vm.go
index dc58f950..ba5c3c3e 100644
--- a/vm/vm.go
+++ b/vm/vm.go
@@ -100,6 +100,7 @@ func (vm *VM) Call(caller, callee *Account, code, input []byte, value int64, gas
 			*exception = err.Error()
 			err := transfer(callee, caller, value)
 			if err != nil {
+				// data has been corrupted in ram
 				panic("Could not return value to caller")
 			}
 		}
@@ -784,7 +785,7 @@ func (vm *VM) call(caller, callee *Account, code, input []byte, value int64, gas
 
 		default:
 			dbg.Printf("(pc) %-3v Invalid opcode %X\n", pc, op)
-			panic(fmt.Errorf("Invalid opcode %X", op))
+			return nil, fmt.Errorf("Invalid opcode %X", op)
 		}
 
 		pc++