Even the most organised movie shoot is going to face its share of problems, because no matter how well-prepared a filmmaker might be, a cast and crew in the hundreds will always result in the unpredictable and unexpected.
Mistakes are simply a part of filmmaking: executing even the simplest scene correctly can require countless takes due to cast members flubbing their lines and various technical issues.
But sometimes a "mistake" occurs in tandem with the work of a great actor and unintentionally accentuates the emotion of the scene, so much that the filmmaker has no choice but to leave the error in.
To be clear, this list won't be focusing on a tyrannical director forcing method acting upon their cast - looking at you, William Friedkin - nor mistakes that were accidentally left in, but erroneous moments that the filmmakers consciously retained in order to preserve a great piece of acting.
In each of these 10 scenes, the actor was so thoroughly in the zone that calling "cut!" or using another take for the final film would've been absolutely criminal...