Plot hole: Kelly is a friend of Jason's from LA and they have been seeing each other since he transferred to Seattle (he mentions 'the other day' she saw something at the pet store). Basically the one passion in life Jason has, is karate. Her brother is the US karate champion, and yet Jason didn't know that or even that he was from Seattle. That's absurd on several levels, since it implies that he wouldn't know her last name (then how did they stay in touch?) and they never ever talked about anything personal, because it's fairly obvious it would come up really easily.
Continuity mistake: Scott white-knights the poor RJ that is being cradled into oblivion by the bulky bully. He jumps over the railing and kicks Scott; obvious is the lack of continuity; the guy with the striped white and green shirt that had folded arms and now is just standing there with his arms low is very obvious, but Scott himself was holding RJ by the thigh and in the second shot, to facilitate the kick, he is holding it by the far end of the leg. (00:22:05)
Continuity mistake: When Scott backs RJ against the railing in front of the burger joint, some fellow bullies in the background change position between shots (the dude with the little ponytail had his left arm raised in guard and it's down, the guy with the horizontal stripes is suddenly with folded arms). (00:21:40)
Continuity mistake: RJ is skating away from an angry Scott he just bit. In the sequence with two construction workers and a ladder, the natural light changes completely between shots (no direct sunlight at the beginning, clear shadows and sunny area in the close-up of the board tricks). (00:16:55)
Continuity mistake: Scott hoses RJ with the window cleaning machine; in the first shot RJ is really close to the fence, but the skateboard slips on sidewalk that is somehow already wet. Then Scott sits on him and begins brutalizing him, with the cement around RJ's head that is wet or dry depending on the shot, and when he gets up it's like they never fought with any water involved, the sidewalk is all dry. (00:16:35)
Continuity mistake: The obese comic relief is mocking his dad's speech as he puts him to work washing windows. At the first cut when he approaches the window you can tell time (and takes) passed, because the wet area on the wall under the window changed size considerably. The wall even basically all dries up the moment Scott spots RJ. (00:16:05)