The Adventures of Bad Boy and Dirty Girl - S3-E7
Revealing mistake: Frasier comes in complaining how workmen in the service elevator have dribbled chili dog on his foot. He is holding out his shoe saying "How ever will I get that stain out", and Eddie runs over and starts licking what we think is the shoe. Frasier makes a comment about dog saliva being the universal solvent. In fact, Eddie does not even touch the shoe, but licks the floor in front of Frasier. Obviously the floor was treated with something Eddie would enjoy licking up.
The Show Where Diane Comes Back - S3-E14
Revealing mistake: When the waitress brings Frasier his coffee, she tilts the cup quite a lot and you can see that it's empty. (00:13:00)
Revealing mistake: Near the end, in Cafe Nervosa, the guy that wants to sue Frasier gives Niles a push. Niles stumbles backwards, knocking over some chairs and the coat rack before falling down, taking the table by the window with him. If you look closely just before he hits the table, the woman sitting there (rather than moving out of the way), very subtly pulls on the table, so as to upend it so Niles can go down with it.
Revealing mistake: Frasier, Roz and Ben are sitting at a table in Cafe Nervosa. However, as the camera is panning out when Ben sits down you can see the outside of Cafe Nervosa because the wall is only a small partition. The camera zoomed out too far. (00:06:30)
Answer: It equals the record of James Arness of "Gunsmoke" for the actor who's played the same part on TV NOT in a soap opera. If you include soaps there are two main contenders - William Roache, who's been playing Ken Barlow in "Coronation Street" in the UK since 1960. Don Hastings has been playing Robert Hughes in US soap "As the World Turns" since 1960 as well.