The movie mistakes are funny. There is supposed to be a crew that does nothing except track scene continuity, like cup color, ties, etc. They normally do a good job, but mistakes happen, obviously.
It's especially hard during filming when they jump about in time. They don't film a movie in the order we see it. They might film the end of the movie first, and jump about filming scenes a bit randomly. It is the job of this crew to make sure everything is the same when the scenes connect.