No fun allowed

In S3A only, if you defeat the Blastoid enemies that are placed directly over the special stage rings in Hydrocity Zone 1, you can actually fall through a hole in the floor directly into the ring's secret room.

For whatever reason, in Sonic 3 & Knuckles, these holes were hastily patched up with invisible collision objects, and as a result, Knuckles can't glide-land or climb up on this ledge properly.

Then again, in S3A you can defeat Blastoids by just walking into them, likely because they don't use the regular enemy collision type. So uh, pick your poison, I guess.