In a moment of pure debugging hell and hate, I put an alert with the text "WTF!" for a condition that should never occur. I could never get it to reproduce and, due to some distraction, I checked in the code with the alert still there.
I probably don't have to mention who got that alert while working one day. The president of the company, of course! Luckily for me it was not a customer-facing function & they found it funny.