Not really bug I 'did', but one I was partially responsible for uncovering the root cause of.
Have you ever gotten caught on a slope whilst rolling in the Sonic games, and then you lose all your speed? Well, Sonic 1: South Island Definitive ran into it as a frequent issue with it's pinch phase for the GHZ2 boss, which involves rolling in a half-pipe to get up to Robotnik.
It was enough of an issue that we actually tried to fix it, because you'd ALWAYS get your speed robbed. So, we'd at first think that CD 1995 fixed it, as such, we went to someone who was knowledgeable with that game. Turns out, we were wrong, but the issue was so simple it was funny: Sonic's collision sensors aren't properly moved up when you roll, since your hitbox changes in height when you roll. This would mean that the game would think you were going into a solid wall when you weren't, therefore stopping you. All it took was shifting the collision sensors up when rolling to fix it.
Fun fact.
Every single Sonic game that branches off from the Sonic 1 engine (aka S1, SCD, S2, S3, S&K) has this bug intact. So that's why you can randomly get screwed over by HCZ ramps. :p