I watched Games Done Quick's video "Community Spotlight: Sonic 3 & Knuckles Edition":

They use the slope glitch to complete Ice Cap. At 21:15 (video time, not game timer), one of the commentators says that if they softlock in Launch Base, they'll get sent back to Ice Cap and this time, they won't be able to use the slope glitch again.

Sure enough, at at about 26:45 (video time) Launch Base is failed, and because there was no save, they get thrown back to the start of Ice Cap Act 1, and have to play through it glitchless.

My question: why can't the glitch be used second time through Ice Cap?

Does something prevent the glitch from activating at all? Or is it to do with which sections of the level are loaded; you can activate the glitch but can't use it to go anywhere useful? Is it the lack of saving at the end of the Zone that causes this - how?

Basically for the slope glitch to work objects need to be in a specific order in memory. Making the top big ring disappear (by collecting it) messes with that order.

Originally we were under the impression that entering the 2nd big ring fixed the order as the stage reloaded, which gave you 1 attempt to retry it.

However, since that stream, we had a discussion on the discord, and Chrezm did some science with it, that meant that you only need to enter the room where the 2nd big ring was to correct the order of the objects in memory. So on the 2nd run through, Ctrl could have entered the room where the 2nd big ring was, leave, and be able to still get slope glitch.