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).
Revealing mistake: When Peter is on the trampoline, instead of saying "Peter," Carol says, "Go get 'em, Chris!" and a few minutes later the same thing happens after Cindy has a turn alone, you can hear Greg say "Why don't you give it a try, Eve?" (00:18:10 - 00:19:50)
Murder Ain't What It Used to Be! - S1-E7
Revealing mistake: Jeannie is not supposed to see Marty at all, but when she is in her flat and she is getting up off her sofa, she is seen walking around him, as if she can see him.
Revealing mistake: After Mr. Maxwell says that the local people refuse to work on the movie, Velma asks why. In her close up, her right pupil is horribly misshapen. Not round, but a wobbly, almost cloud like shape. (00:01:35)
Revealing mistake: When the drunk Olive is standing on the chair lifting her skirt up it's obvious that Michael Robbins the actor who plays Arthur is trying very hard not to laugh.