Blade's girlfriend throws a cigarette at her side, while she's crawling on all fours in the air duct, to find a safe way out of the school. The same scene is repeated after a while (the cigarette's still there, it's exactly the same shot!) when she's supposed to have been crawling far from her starting point.
In the second scene where the baseball player is told to go fetch the ball, said ball is thrown into the corner of the gym hall. However, even before he walks off to go get it, the same ball has bounced off the wall and returned, not only in his direction, but past all the characters including himself. Despite the ball now in the opposite side of the room, he still walks off to where the ball was thrown.