after renton has had sex with kelly, and she tells him he cant stay with her, you see he has no condom on, yet when he is pushed out into the hallway, you see him pulling the used condom off
The Object Renton takes off is actually a rubber band this keeps his errection up for longer as using heroin can cause failure to "keep it up" for long periods of time :) im sure it said somewhere in the film about that.
Before they have sex, she throws him a condom. Out in the hall, he is clearly taking it off. Watch the movie again. He's pulling it by the tip and it snaps off of him, a very messy way to remove a used condom, by the way. This is definately NOT how you would remove a c*ck ring. I've had the unfortunate experience of watching this scene over and over again in a room full of die-"hard" fans and I think this is a slip-up, but, God help me, I'll look again. The things I do for you people...