Continuity mistake: Zack states to his father early in the movie "I've been in love with Kelly since grammar school" yet late in the movie during the wedding, Mr. Belding tells other guests in reference to Zack and Kelly "this is special. They met in my school" (a high school). These two stories conflict one another.
Continuity mistake: The layout of Zack's room is different in the beginning of the movie than the TV show. In the TV show, his door is to the left of the window and in the movie, his mom walks in from the right of the window.
Continuity mistake: In part one of the Las Vegas wedding A.C comes out of the water after retrieving a golf ball and you can see his shirt is wet on the bottom 2 inches or so. He is sent for more balls and upon entering the pro shop you can see that the wet part of his shirt has jumped up almost to his armpits.