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.
Plot hole: The house has no pool, even though Walter tells the homeowner she has the best kept pool he's ever seen.
Suggested correction: When Walter tells the homeowner that the pool is well-kept, she lets out a groan that suggests that it's a flood.
I think it was because the lady who owned the house said her husband was deported out of the country because it turned out that during the war her husband was Hitler's pool boy, and she didn't know it. I would assume that he would keep a nice pool and since her husband was gone, she needed to sell the house.
Plot hole: When the bullies take revenge on Marty, they doctor the chemicals he is preparing so that they explode. The bullies were punished some time in the afternoon, judging by the sun in the very first shot. Their detention is at 4:30 in the afternoon. Somehow, they manage to acquire the necessary chemicals for the explosion within a couple of hours, even though they are stuck at school. In addition to this, they have no way of knowing what chemicals Marty is preparing, so even if they were able to get chemicals, they would not know which ones to get to destroy his experiment.