Whether it’s a car engine or an operating system, anything with several thousand parts can develop the occasional technical hiccup. OS X is far more resilient than its predecessors, but it’s still a complex system with the potential for occasional glitches.
Most freaky little glitches go away if you just try these two steps, one at a time:
Quit and restart the wayward program.
Log out and log back in again.
It’s the other problems that’ll drive you batty.