Interesting video displaying movie “screw ups” that, as mentioned in the title, wound up being in the film itself:

The last one mentioned, that from Back To The Future II, is one of those stunts that went very wrong and wound up being used in the film.

One of the more spectacular stunts featured on Mad Max 2 aka The Road Warrior, featured a motorcycle driving punk hitting a car and then being spun around into the air.  This was not intended (the stuntman was supposed to release the motorcycle earlier to have a more controlled flight) yet made the film’s final cut: