The Halting Problem

But Turing also went further, showing that there were certain things that even the most powerful machine could not compute. In particular, he pointed out, no computer could predict its own behavior… The “halting problem,” as it’s known, is hopeless… Except in very special cases, the fastest way to find out if your program will take a given action is to run it and see.

The Dream Machine, by M. Mitchell Waldrop