Continuity mistake: Frasier meets Heather in the bar. She is reading a book which she has propped open on the bar, but as they talk, she takes it off the bar and closes it in her lap. A couple of shots later we see book is back on the bar and she is closing it again.
Continuity mistake: Frasier is just saying goodbye to Niles and we can see the brochure for Daphne's reception to the left of his sherry glass. When he stands up and shouts "Daphne!" the brochure has moved about 6 inches to the right, even though no one touched it.
Revealing mistake: In the scene where Frasier and Lilith meet in the living room, where Frederick has created a romantic atmosphere, Frasier has a long speech about how Lilith is not going to get him back, ending with "I need to be finessed!" and Bebe Neuwirth goes out of character and begins to giggle, before she quickly covers her mouth and pulls herself together. Though Lilith might have found Frasier's comment amusing, it would definitely not be in character for her to giggle like that.
Deliberate mistake: In the pilot episode, "The Good Son," Frasier has a telescope which is seen on his balcony. The telescope disappears for the next 3 episodes but in Ep. 5, Frasier gives his father a telescope as a gift. It remains as a prop for many seasons.
Continuity mistake: In the 8th episode of season 11, "Murder Most Maris", Frasier claims to have been "punched in the face yesterday by a man now dead". When he says this, two nights have passed since the murder. The next day Niles panics at the café and then sleeps for 20 hours. This time Frasier claims he was punched in the face three days ago by a man now dead, when by now it has been at least four days.
Continuity mistake: Roz comes back to the restaurant table, after phoning her babysitter, and says, "I have to go". At this point her purse is over her wrist. As she finishes saying, "Alice got a bead up her nose", she is holding the purse by its strap. A few shots later it is over her wrist again, even though she never moved it.
Continuity mistake: Frasier is off to find Niles and he tells Martin to come with him. Marty puts his chair down as he says "I don't want to go to Niles'" - the shot changes and we see him lower the chair again. His coat in his lap also changes position between these two shots.