> The other half is "but I do know the answer

Except you don't!

If you claim to know the answer you've made a grave mistake and fooled yourself.

If you ran the code in a compiler and used that to conclude "this is the answer" rather than "this is an answer" then now is a great time to learn how easy it is to fool yourself. You just need you ask yourself what assumptions you made. I'll wager you assumed all compilers process this line in the same way.

Or just RTFA, or Susam's, as that's exactly what they are about. They explain why this is undefined behavior.

  | The first principle is that you must not fool yourself — and you are the easiest person to fool.
  - Feynman