I've definitely experienced some scary bugs caused by runtime bytecode injection (from a tracing library). I didn't bother tracking it down further than identifying it was at the intersection of some slightly strange type and the tracing. Caused VM errors when this method was called but only when the tracing was configured.

Not saying it's not interesting to experiment with things like that, but there are non-trivial associated, if you ask me, to depending on it in production.