I heard of someone who gave a speech once showing math errors in commercial movies. I've never found out who gave that speech, but I was intrigued by the idea and after seeing this humorous mathematical bit from a Ma & Pa Kettle movie, I've been on a search for more. This one I love to show teachers as an example of a great assessment item. Just show it to a fourth grader and ask what's going on to get a clear idea of their understanding of multiplication, division and....well I don't want to give it away.
Two days later: At the Asilomar CMC Math Conference and Gary Tsuruda showed this clip in his keynote. This clip is from and Abbott and Costello movie, but the math part of the script in both movies is almost identical.