Continuity: When Paul goes to get pizza and a movie for Dora, the movie messes up the order that he gets them. First, he goes to the pizza place, and picks up a pizza, you can clearly see him holding a bag. Then, he goes to the video store (with a cameo by David Spade), he has nothing in his hands. Then, he has both after.
Great sites
Mistakes
Dora is sleeping after being taken to the hospital, she wakes up in Paul's bed. Assuming, she recognizes the place as the one where the fundraiser/party is held, how can she know that it was Paul's bed and that he lived there? Furthermore, she also seems to know that she was in the hospital, and that Paul was the one who took her. Seeing the exchange of dialouge between Paul and the doctor, and Paul and her, there was no way she could have known that information prior to her waking up. See more...
Loser mistakes
Factual error: When Dora and Paul walk into find the newborn baby kitten on the floor, it shows what a kitten would actually look like if it was newborn. However, after they save it and are with all the kittens later on, the kittens are walking around and they have their eyes open. Kittens don't open their eyes or walk for at least 2 weeks after being born. In the movie, they had just been born.
Other: When the waterbed on the bunk above Paul's bed begins leaking and he gets up to hold it, you can see that the water is not actually coming from the the mattress but somewhere, like a hose, concealed by the front wood panel.