I decided to dick around with the demo bug in Sonic the Hedgehog
(REV00) today (if you hold C, you can cancel out Sonic's jumps, resulting in hilariously impotent demos - this is fixed in REV01), when I got wondering: is it possible to collect the Chaos Emerald in the Special Stage demo?
After half an hour or so of dicking around with it, I can confirm that the answer, amusingly enough, is "yes". Let the demo play through for the most part, such that he's attacking the side closest to the entrance, but once he hits the R on the corner of the goal, hold the jump button such that he falls back down toward that side (if you do it to late, he jumps off the R and winds up on the complete opposite side instead, as in the normal demo). I haven't really worked out a consistent strategy after this point, admittedly, but Sonic can accidentally wind up grabbing the emerald from here. The demo just ends abruptly, though.
Amusingly, future Special Stage demos will now start in SS2. Selecting "Special Stage" from the level select puts you in SS3, and failing it confirms that you have the emerald the demo Sonic grabbed. Starting the game normally, however, resets your emerald count back to 0.
Just a cute little tidbit. If you'd like to take a look at the effects without doing it yourself, I have a Kega Fusion 3.64 save state here