Games in their early stages, like the one I'm working on, sometimes develop wacky idiosyncracies during development.  Here's an amusing example... for many months now whenever someone would take a screenshot from within the editor, one of our units would appear prominently in the final image.  This alone might have been amusing, but as it turns out, the unit would always appear stripped of any techs the player had researched.  Due to how we represent the player's military units, that means he wouldn't have even his baseline civ-specific armor so he would appear, well, mostly naked.  Fortunately his modesty would be preserved by a pair of underwear that was painted into the base mesh.

Thus was the legend of "Underwear Man" born, making his ubiquitous presence in every screenshot we would make for internal distribution.  He was, as you might imagine, the subject of much amusement.  But all good things must come to an end, and today I finally fixed the bug, banishing Underwear Man forever.

I think my favorite bug of this form was back on AOM.  For a few weeks, at some random point in the game whenever you would train a new unit, all your existing units would identify it as an enemy and immediately turn on their new comrade, murdering him as soon as he exited the barracks.  There was something just so very wrong and yet amusing about watching your units act in such a twisted manner...