I died on the Chemical Plant boss (It's been years, my skills are rusty), and when I came back, I couldn't go on that loop - the left side's collision must have gone wonky. It may have been caused by Knuckles, because I switched to him before trying to run up the loop.
Doesn't appear to be in the bug list.
EDIT: Sonic and Knuckles couldn't seem to go through the loop, but I switched to Tails and he did it fine after I flew around and collided with some stuff, albeit with some layering issues - he went behind the loop instead of in front of it, Layer wise.
EDITKING: Strangely, the Speed Shoes don't seem to slow the music down in Aquatic Ruin zone.
EDIT AGAEN: The Pinball flippers and launchers don't seem to work unless everyone is present. Wait, never mind - sometimes they just don't work.