Revealing mistake: On Billy's first day of school when he is outside waiting for the bus and it just passes him you hear the noise of kids on a bus, but when it passes no one is in it.
Billy Madison (1995)
Ending / spoiler
Directed by: Tamra Davis
Starring: Adam Sandler, Bradley Whitford, Darren McGavin, Bridgette Wilson-Sampras
Billy has a academic decatholon competition against Eric. The winner gets the company. Billy is winning and when Eric is given a tough question, he loses it and pulls out a gun. Billy is saved by the former classmate guy with the "People to kill" list, who shoots Eric in the butt. Billy graduates from high school, decides to give the company to Carl and go to college.
KLF
More mistakes in Billy Madison
High School Principal: If there's any attempt by either contestant to cheat, especially with my wife, who is a dirty, dirty, tramp, I am just going to snap.
Trivia: The pool that Billy appears in is of the exact same set of pools Navin R. Johnson [Steve Martin] used in The Jerk.