'Dentist on the Job'
This is an old 1960s British comedy movie which starts playing when you play the Holy Grail movie, but then someone is heard over it saying "This is the wrong bloody film! Here, change the reels..." And then it starts with the Holy Grail movie proper. It's just a funny example of Python humour.
The funny thing is, I lent this DVD to someone in the next room, and they came back complaining that I'd given them a dud disc! I told them to carry on watching, they did so and saw the joke. I think it's a funny anecdote, and if it happened to me, it surely happened to others.
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