It's in the bit just after Sidney sleeps with her bloke for the first time. As we all know, there are two killers in Scream 1, her boyfriend being one of them. This being so, there should only be one other killer on the loose as she is with her boyfriend, one of the killers. As it happens, one killer enters the room, meaning that she is unknowingly in the room with both of them - one masked killer and her boyfriend.
Then when she leaves the room, there is another masked killer outside - giving us a total of three killers in one scene. How many killers are there supposed to be - and how did no one notice this before?