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: Niles, Frasier, Marty, and Daphne are at Niles' apartment and they have just found out Maris killed her lover. Niles' overcoat is on a chair behind the couch. He decides to go the police station, tells Daphne to stay put, and heads towards the hall on the left. In the next shot he is carrying his coat and heading out the front door with Marty - there just wasn't enough time for him to have got over to his coat and back to the door.
Continuity mistake: In Cafe Nervosa, after Niles has taken off all his clothes, Frasier wraps him up in a couple of Cafe Nervosa aprons and Roz picks up Niles' clothes from the floor. In one shot, we see his tie towards the top of the pile in Roz' arms, and in another it is swinging free at the bottom of the pile. The clothes change position in her arms from shot to shot. In one shot the boxers are still on the table, with a newspaper. In a subsequent shot, Roz has the boxers. When Niles is being taken outside, the table nearest the door has only a cup on it. In the next shot, we see the newspaper on the table again.
Continuity mistake: There are several bad cuts between shots when Niles and Frasier are on the balcony. Keep an eye on the height of Niles' sherry glass. The most noticeable occurrence is when Niles mentions "detachment" and the shot changes. He is suddenly holding his glass at least a foot higher in the air.David Mercier
Revealing mistake: At Cafe Nervosa, after Niles has had a mini-breakdown, he has shed his clothes, boxers and all, all over the place and is supposedly naked in the corner with only a newspaper for privacy. When Frasier has him stand up to wrap aprons around him, one can see that David Hyde Pierce is actually wearing light-coloured briefs.