i was playing sonic 2 with knuckles in it and i did the special stage and got an emerald. then after i died and i came back at the checkpoint. but i had all my rings and i still had 3 lives. it happened like 3 or 4 times. is it a glitch??
If it is, it's a very cool glitch. 😀
I know I've come across a few not very nice glitches in the old Genesis games. One of them had me falling through a wall in Hydrocity Zone, running around in said wall, before plummeting further into said wall and plunging to my inevitable death at the bottom of the screen. That was not a nice glitch.
Though this isn't a Sonic game, the strangest glitch I've come across was in Paper Mario o.o Started a new game, walked into the room where you see Peach the first time, then all of a sudden all sorts of code rudely interrupted the gameplay and music to the point where all you could see were little bits of scattered Mario on a pink backgrounded screen with fragmented code and an annoying flatline noise to top it off.
So yeah. Be happy you have a nice glitch 😀
~Shadowed Spirit Sage
AFAIK, yes and no. See, they wanted it so you kept your rings after leaving the special stages, a la S3&K; it just so happens that they forgot to make it so death erased that.
Regardless, it's part of the game, so abuse it to your heart's content.
I noticed this when I played it.
After leaving a special stage in Sonic 2, sonic has no rings but all the rings appear back on the stage.
After leaving the special stage on Sonic and Knuckles nothing changes.
So, as Knuckles is ripped from Sonic and Knuckles, he keeps his rings, while the stages do what they are supposed too: put all the rings back.
Making for a massive ring bonus.
Why you keep them when you die is a mystery to me though.
I'm not sure if they're conistant glitches, or actual changes to the game, but there are quite a few definite inconsistencies with the games. Knuckles, of course, always keeps his rings after a special stage. His barriers are gray, instead of blue. I could probably go on, but the rest escapes me. I honestly think it was on purpose, and that Sonic Team recoded something when Knuckles was ported over.
But I could be completely wrong on the subject.
~Nytloc Penumbral Lightkeeper
Another funny thing I noticed was something in the background that was supposed to be blue. You change into super sonic and in starts flashing gold. You bring knuckles over and it is red and looks very odd. But I cant remember what it is!
Most of the Sonic glitches I've experienced have left me having to reset the game. 
