How about a bug that turned into an easter egg?
I was lead engineer on a 3-D submarine sim a few years ago. We had the graphics code running, but the engineer who was working on the physics was still tuning his equations. Handling surfacing was particularly hard – it’s a boundary condition where all the equations abruptly change so he’d been sweating over it for a couple of days.
Finally, late one night he called a bunch of us into his office for a demo of surfacing behavior. He ran the sub up to periscope depth and slowly adjusted the bow planes. The boat gracefully surfaced, and the camera popped from underwater to air just like it was supposed to. It looked like a Navy commercial. We all cheered.
Then I said, “What happens if you do an emergency blow?”
“I don’t know,” the engineer said, “I haven’t tried that yet.”
He submerged again and ran the sub down to a few hundred meters. Then he blew all the ballast tanks, set the bow planes to maximum, and gave it full throttle for good measure. The submarine rocketed to the surface.
The boat broke the surface at about a 30 degree angle. The camera popped from underwater to air. The bow of the boat lifted completely out of the water. Then, much to our surprise, the stern of the boat also lifted completely out of the water. Still at a 30 degree angle the entire sub floated gracefully up into the air and off the top of the screen.
We all fell on the floor laughing. We were still laughing two minutes seconds later when the sub fell back onto the screen, splashed down nicely and resumed normal operations.
Subsequent experimentation revealed that airborne operations were actually quite an effective tactic. It made the enemy destroyers’ depth charges almost worthless, while you could still launch torpedos with deadly accuracy. (They’d drop down into the screen a few seconds after launch, hit the water, and home in for the kill.) If you were lucky you could even kill a helicopter with one.
Eventually the physics got fixed, but we kept “Zeppelin mode” in as an easter egg.