Artists would have you believe that every brush stroke is intentional, but it's harder to keep track of them all when you're making a film and working with countless other people spread across multiple departments who you have to trust not to mess anything up. They can often get pretty sloppy with their strokes.
Most of the time, when someone messes up and it makes it into the movie, it's only discovered later when the whole thing is completed. Because why would a film-maker release a film with obviously broken bits in it? This isn't the video game industry we're talking about.
But occasionally, that's actually exactly what happens. Sometimes things that appear to be mistakes were actually intentionally put in there - or left in - because they served the film or improved the shoot in some way....
9. Hagrid's Moving Hut - The Prisoner Of Azkaban
When Alfonso Cuaron took over from Chris Columbus for the third Harry Potter film, he had a different vision for Hogwarts than his predecessor and he set about trying to recapture more of the spirit of JK Rowling's books in his design.
Unfortunately, he also moved Hagrid's Hut and the Womping Willow, which fans have long-considered a glaring mistake (especially as Rowling's own design notes for the films had them far closer to the castle). But it wasn't a mistake.
For the key Buckbeak rescue sequence, Cuaron needed Harry, Ron and Hermione to have a raised vantage point both for them to witness the events and for the time-travel element to work convincingly. So he was forced to move the Hut to suit the story.
Here's Cuaron explaining the shift...