The Spanish Inquisition - S2-E2
Revealing mistake: At the very end when The Spanish Inquisition bursts into the courtroom, you can see that Biggles is not played by Terry Jones (as he is playing the accused judge in the court sketch).
The War Against Pornography - S3-E6
Revealing mistake: When John Cleese (as Long John Silver) falls over after being shot with a tranquilizer dart, you can see his real leg, which was tucked behind him when using the peg leg. (00:20:10)
Revealing mistake: The Italian evening class runs from 7 to 8 PM. The clock at the back of the classroom reads 10 to 2.
Revealing mistake: Near the end of the Bishop sketch, when they use the man as a battering ram, you can see the entire wall behind them tip as they smash through the door.
Scott of the Antarctic - S2-E10
Revealing mistake: In the last shot, the familiar 16 tons weight is broken at the top, showing that it is in fact not a solid weight, but a hollow box.
Chosen answer: That would be Terry Gilliam, who played a lot of non-speaking roles in the series, and some speaking ones like Cardinal Fang in 'The Spanish Inquisition.' He did all the animation as well, but I'm sure you knew that.
rabid anarchist