I was making this code, right, and to test it I kept giving it the input 'test'. Then after a while I decided to give it the input 'foo', and it showed a stack trace.
Any other inputs besides 'test' and it gave a stack trace. It knew what 'test' was, but it didn't understand anything else. I confused my poor little program by showing it that there are other words in the universe besides 'test'.
From now on I'm going to think of them as little humans. That's what they are, really.