Taylor's tattoo disappears at the end of the movie.
When Zack takes Laney to the beach, you see her on the beach with her art book just before she goes to play volleyball. When she leaves the beach and walks back to Zack's car, she doesn't have her art book with her.
The lifeguard tower at the beach jumps from being open and occupied to completely closed up between shots.
Laney's hair suddenly appears long again after she has had it cut.
When Taylor was talking to Zack about meeting Brock her hair style changes several times.
Simon is wearing bi-lateral hearing aids. If he removes them for sleeping, how does he hear Laney knocking?
When Zach looks at his college acceptance letters, the inside addresses shown are those of the colleges. In a real letter, the inside address would be that of the recipient. The school's address would be shown elsewhere on the letterhead.
When Taylor dances by the pool and her back faces the camera it is clear that the actress that is dancing is a double and not the actual actress.
When Taylor is walking with her friends she talks about being prom queen but during this "speech" her mouth does not match what she is saying.
When Preston and Katie are posing for their prom photo the camera pans down and reveals the fluorescent marker tape on the floor.
When Zack's parents come home and Zack's father is checking the mail, the boom is visible in the clock next to him.