Deliberate mistake: During the scene where Richard is playing croquet on the lawn in front of the Delta Iota Kappa house, the letters of the fraternity are visible on the front of the house. The Greek letter for "Delta" should be a triangle yet it is the letter "D" in the shot. This is presumably to follow the "DIK" joke that would not be as obvious if written in correct Greek letters.
Continuity mistake: In the last scene, at Van Wilder's party, Gwen is talking to Van Wilder and her bathing suit strap is on her left shoulder. When the shot changes, it has disappeared from her shoulder and is on her back. She wouldn't have moved it as her hands don't move in the scene.
Van Wilder: You might want to write that down.
Trivia: The titular character of the movie is based on comedian Bert Kreischer, specifically a Rolling Stone article written about him when he was in his sixth year at FSU and told the story of "the top party guy at the top party school." The writeup eventually become a movie concept that Oliver Stone acquired the rights to, but the deal fell through and the writers sold it to National Lampoon. Kreischer himself was not involved in the movie, however.