Once I've presented an application with graphical interface to a client. He gave it to his colleagues to play around, and one of them found a bug. He discovered, that if a user makes a certain sequence of actions, one button from the interface surprisingly disappears.
When the first dude have demonstrated me that effect at the next meeting, and I saw how the button disappeared, I started to laugh as crazy... I honestly tried to get grips on myself and stop, but when I recreated the sequence by myself and saw it again, I started to laugh even harder! I laughed for a several minutes, until I found a bug (it was super-easy to fix, actually) and recompiled the application right there...
Not saying, that my reaction really surprised and perplexed that dude. Usually you don't expect this when you invite an employee to discuss the work... 
I generally find many visual bugs in interfaces/video games so funny, that I can't stop laugh on them, even if it's my fault. Most of people around don't understand, why do I laugh on them too hard, and I don't even know, how to explain.
Do you find some weird things funny, which other people don't?