Ward Bond had an accidental discharge during the filming of The Searchers. Well, kind of... Wayne tosses Bond a Colt, saying "it's loaded". Bond had his finger on the trigger and he fired a blank at hip level. You can see the surprise on his face, but the director kept the scene in the film.
Alan Ladd had an accidental discharge in the final shootout scene in Shane. He was careless with the trigger and fired a blank at the floor of the saloon. Nonetheless, the actor on the high balcony dutifully performed his death scene....
In both cases, the revolvers were pointed in a safe direction (at no one) and were verified as loaded with blanks.
