Older open matte films are chock full of things that were never meant to be seen. Some of the knocks against the infamous Plan 9 From Outer Space, (mike booms in shot, the cheap wooden airplane controls) are actually invisible matted or projected at 1:85.

IIRC, the later Muppet films were hard matted to avoid seeing arm sleeves below a character's waist, and the tops of performer's heads on tv and video.

The "endless bicycle chain" scene in Pee Wee's Big Adventure was used to explain matted widescreen in some 90's Laserdisc store flyers.

http://www.youtube.com/watch?v=3iD-UoBg-YU

My favorite open matte "blooper" is the scene in Bachelor Party where a character wrapped up in bedsheets falls out a window. Two very visible stagehands are at the bottom of the screen to catch the actor.