I was bored once and put an error message where no error should ever occur, curious to see if it would ever occur, and made it just nutty enough that I was sure engineering would hear about it if it happened.
Three years later a hardware fault triggered the impossible in a newly installed customers system:
Another one bites the dust >CLAP!< Another one bites the dust >CLAP!< Program fall down and go BOOM! in line #245
Neither the customer, tech support nor the salesman were amused at the time but did chuckle about it over a beer later.
HOWEVER, there was a HUGE upside. Since the impossible message got displayed we knew it was something serious real quick and it probably saved us a ton of time figuring out the problem versus trying to track it down via the normal error traps typically displayed. Turns out the CPU was actually overheating because someone had blocked the ventilation and the computer was roasting itself.