I saw a movie in a theater years ago where the projectionist did not change the lens and/or mattes when switching from previews to the main feature. The projected image was much taller like a 1.85 or a 1.78, but the feature was shot wider 2.35/2.40. However, the image wasn't protected for the taller aspect, so all of the on set gak could be seen as it was never meant to be shown at the taller aspect ratio.

I've also sat in transfer sessions where the Pan&Scan decisions were being made to transfer wide screen down to 4:3 vs just doing a center crop extract. It makes you appreciate just how much effort is needed when done as best it could be rather than just the fast/lazy way.