I found a discussion of this elsewhere, so let me touch on a few things that have become legendary and are not true.
1) 1985 World Series Game 6. Don Denkinger cost the Cardinals the World Series.
Not true. Denkinger did make a horrible call, but he didn't drop the foul pop, didn't have the passed ball, didn't give up the following hits, and didn't lose with a 20-game winner on the mound in Game 7.
2) Colorado - Missouri 5th down play in 1990. Not only was it one of, if not the worst blown call in football, Colorado actually won a national championship that it didn't deserve.
A horrible call, yes. Going a bit far with "national championship it didn't deserve," though.
OTHER POSSIBLE MENTIONS
1995 Iron Bowl - let's face it, Curtis Brown was in
1995 Arkansas - catch, are you kdding me?
The mugging of Roy Jones, Jr. by Korean boxing officials in the 1988 Summer Olympics (absolutely true)
2012 Braves vs Cardinals - that's an infield fly????