There are a handful of situations that the C++ standard attributes as undefined behavior. For example, if I allocate with new[], then try to free with delete (not delete[]) that's undefined behavior - anything can happen - it might work, it might crash nastily, it might corrupt something silently and plant a timed problem.
It's so problematic to explain this anything can happen part to newbies. They start "proving" that "this works" (because it really works on the C++ implementation they use) and ask "what could possibly be wrong with this"? What concise explanation could I give that would motivate them to just not write such code?
"Congratulations, you've defined the behavior that compiler has for that operation. I'll expect the report on the behavior that the other 200 compilers that exist in the world exhibit to be on my desk by 10 AM tomorrow. Don't disappoint me now, your future looks promising!"