Byteman uses an Object[] to pass local/parameter values into the rule execution engine and uses the same array to retrieve values updated by the rule code which it expected to write back into the trigger method's local var slots. Obviously returned values are retrieved from the array with Object type. If the local var slot does not hold an Object then the injected code needs to include a checkcast and, in the case that the slot stores a primitive, unboxing code. It seems that in some cases the wrong type is used for the checkcast leading to a VerifyError.