Factual error: Probably an in joke - the wheels of the "airliner" shown landing at LA international airport are those of a B52 bomber. They are very distinctive and look nothing like those on any airliner ever made.
The Man Who Broke the Bank at Red Gap - S2-E17
Factual error: An egotistical, prissy and dishonest lawyer like Mr. Fletcher wouldn't make the mistake of allowing his name to be misspelled on his office door. It reads "Winfred Fletcher." His first name, as it's pronounced throughout the episode, is "Winford." (00:26:50)
Edith's Final Respects - S9-E5
Continuity mistake: In this episode Jean Stapleton plays two characters. As Edith Bunker her earrings change from gold that curve below her lobes to white round ones later in the show.
Is Lupin Burning... ?! - S1-E1
Other mistake: The logo of the evil organization, Scorpion, contains just a little misspelling. It's missing the R. (00:02:50)
Take Seven - S1-E3
Visible crew/equipment: When Danny is walking towards the cabinet with the alka seltzer in it, you can see a white cloth moving fast in the left upper corner for a second. (00:13:30)
Revealing mistake: During the opening theme, when the children are watching the magician, you can tell they were not actually filmed together. The magician scenes are filmed more professionally using a sharper tape or film image, whereas the children scenes look more like a standard fuzzier film-type such as on a projector (primarily used at that time).