One of the comments on this website is that Roman is the killer in Scream 3 but Gale checks his pulse and he doesn't have one yet he is just faking his death.

But in the original draft of the movie the makers had a seen where he explained that by saying that if you put pressure on your armpit it stops your pulse but they decided to cut it out of the movie because it dragged on to long.

So the slip up is they forgot to cut the scene out where Gale checks his pulse