I got lucky and found a glitch on Riptide Relic 1 where I had infinite attempts at the Special Stage. When the SS finishes you either end up back at the lamp post with the stars spinning above it, or earlier in the level with a 50+ ring count. And yes, the special stage IS hard and I'm so glad I discovered that glitch because playing as Super Sonic is awesome... although it is a little weird to see SS using Sonic 1 walk frames while normal Sonic uses 2s (not that it's a bad thing - he looks cool!).
Oh, and Vexer... PLEASE don't fix it, it served me very well.
There are some minor game-breaking bugs as well. I've found those two so far:
Going Supersonic while riding the minecart (the one in that Mine Zone whose name I don't recall) left me hanging in the air, unable to move.
The first transportation pipe in Molten Meltdown Zone act 2 left me stucked halfway, unable to move once again (sonic regains the ability to jump, but it's useless).
In both cases, one must patiently wait for a "time-out death" in order to keep playing, so I think they should be worth fixing...