Revealing mistake: In the scene in which Ryan Dunn jumps into cacti with his BMX bike Preston Lacy is supposedly yanked off of his couch by the momentum of Ryan's jump. However, if you look closely in the background it is obvious that Preston Lacy hardly moves at all from Ryan's jump and the effect was edited in later. [In reality, Dunn barely even movied Lacy. The crew reshot it and pulled Preston off the couch by hand.]
Revealing mistake: Now it is said by Dunn that at the end, when Dunn craps out the toy car, it is shown again in the credits, but is put in reverse so the shot looks like it was filmed at a different angle.
Chosen answer: It's Carl Orff's classic composition "Carmina Burana". It's most prominent use was in Excalibur (1981), other movies to feature it include Detroit Rock City and General's Daughter (both 1999).