10 More Movie Mistakes That Became Canon

Who can forget the scene from The Sound of Music where Captain von Trapp (Christopher Plummer) gets into a heated argument with Maria (Julie Andrews), as she encourages him to pay more attention to his children?

Von Trapp tries to shut the argument down, but when Maria insists she isn't finished, he retorts with, "Oh yes you are, Captain," before rolling his eyes and correcting himself with "Fräulein."

It obviously doesn't make much sense for von Trapp to call Maria "Captain," and as it turns out, this was a genuine flub on Christopher Plummer's part. 

And yet, because director Robert Wise felt that it fit the scene perfectly, and Plummer was smart enough to not break character, he used it in the final film regardless.

Ultimately this mistake only makes von Trapp look more bothered by the truth bombs Maria just dropped on him, so it's a perfect - it unintended - accompaniment to the scene.