Star Wars – A New Hope
This might actually be one of the most famous extra mess-ups ever – so famous that the movie was retroactively changed to accommodate it. So, a bunch of stormtroopers manage to track down C-3PO and R2-D2 on the Death Star.
As they enter the room the droids are in, one of the troopers very, very conspicuously bangs his head on the bottom of the blast door.
Twenty-seven years later, when George Lucas was remastering the movies for a special edition release, he added a ridiculous “bonk” sound effect to accompany the blow. That’s almost as funny as Jar Jar! Almost.