Barry sits down on the middle cushion of the couch next to Bill, who is sitting to his right next to the arm of the couch. Bill then gets up to get another beer. The shot cuts to Barry's dad who is sitting in his recliner and from the angle of the camera, you can see the end of the sofa where Bill was sitting. Barry is now sitting on the end of the sofa where Bill was sitting w/o enough time having passed for him to have moved over.
Right before Barry jumps into the water hazard to look for Bill's driver, the bottom half of Barry's jeans are already wet and dirty. This was probably a later take of the scene, and actor Troy Gentile didn't change his jeans between takes.