Goof (incorrectly regarded as a mistake): As always with any long-running comedy series, there are some inconsistencies in characters' back-histories between episodes, but we generally excuse them. Discrepancies between this series and Cheers (from which it is a spin-off) are completely exempt from nitpicking (and some have been explained away, anyway).
Goof (factual errors): In the 2000 season premier, Donny, a Seattle-based lawyer, launches a breach of contract lawsuit against Daphne Moon for backing out of their wedding. It is noted that the suit sought "$100,000 in punitive damages". Washington State does not allow for punitive damages in its courts. (Only cases heard in Federal Court are eligible for punitive damages in the State of Washington.)
Goof (continuity error): In the episode where Marty Crane is made President of the condo board over Frasier, Roz visits Niles and Daphne at their home. They offer her fruit salad. Towards the end of the scene, the amount and type of fruits in Roz's bowl changes between shots.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): Contrary to what Gil says in "Ham Radio", there are no fens in Surrey. They are in Cambridgeshire, Lincolnshire and Norfolk, over 100 miles away.
Goof (continuity error): Throughout the scene where Martin brings out the new chair in "Bla-Z-Boy", and then Frasier brings out Martin's old chair, Daphne's legs keeps changing positions between shots. In some shots her legs are crossed, and in others, they are uncrossed. There also is a similar editing goof involving Daphne's legs in "Dark Victory".
Goof (revealing mistake): When Niles drops the ring in "Goodnight Seattle", it can be seen bouncing out of camera range and not into the plate, where the plot dictates it should have fallen.
Goof (continuity error): In "Goodnight Seattle", when Niles drops the ring, Eddie is eating from a plate. On a pulled-back shot, Eddie has licked the plate clean; but on the close-up of Eddie, the plate still has food on it. On the next away shot, Eddie is still licking a clean plate.
Goof (factual errors): In the "Frasier Crane day" episode, Daphne phones the INS and bothers the Mayor of Seattle about her passport, which would be a job for the nearest British consulate.
Goof (continuity error): In Season Two's "The Friend", when Bob Reynolds wheels away from the table in Cafe Nervosa and out the door, he has the book on barbecuing on his lap. A split second later when Frasier exits behind him, the book is back on the table.
Goof (errors in geography): The view of the Seattle skyline from Frasier's high-rise condo is an impossible view. The angle of the view, with the Space Needle in foreground and the skyscrapers in the background, is from north of downtown Seattle looking south. There are no high-rise condos north of the Space Needle, just one very massive hill, Queen Anne Hill. In order to get the view that he has, Frasier would need to have a ground-level house about halfway up the side of Queen Anne Hill.
Goof (continuity error): Frasier flashes back to his first day on the air three years earlier. At that point, he had longer, curlier hair in the first season. Yet in the flashback his hair was its current short length.
Goof (continuity error): In some episodes, the Space Needle can be seen in the view from Frasier's apartment. In other episodes it's missing completely.
Goof (factual errors): In "Our Father Whose Art Ain't Heaven", Daphne is asked if she is cooking another English delicacy. She replies yes and says she has got a calves stomach from the butchers to make haggis. Haggis is Scottish not English and is made with a sheep's stomach.
Goof (continuity error): In season 7 Daphne's Manchester accent becomes more prim and clear, while throughout all other seasons before and after season 7 her accent is high-toned, thick and whiny.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): The series context is that Frasier's dad is an ex-cop and Frasier became a psychiatrist despite his father's desires. In Cheers: #8.9 (Two Girls for Every Boyd), Frasier reveals that his father was a scientist and wanted him to become a psychiatrist; but he was reluctant to do so.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): Niles guesses from Daphne's accent that she is from Manchester and she says he is right. In fact Jane Leeves's assumed accent is not identifiably Mancunian but appears to be a generic Yorkshire.
Goof (continuity error): As soon as Frasier gets up from the piano after having been stared at by Eddie the Dog, when the camera cuts to a wider angle, Eddie has disappeared.
Goof (audio/visual unsynchronized): At the beginning of the cocktail party the pianist's hands doesn't sync with the sound being heard.
Goof (continuity error): In the beginning of the episode, Frasier is having great difficulty understanding the caller when she says the word "monsieur". However, various other episodes of the show portray Frasier as clearly fluent in French.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): When announcing the results of his psychological inquiries, Frasier says "the crinimal (c-r-i-n-i-m-a-l) among us can only be..."
Goof (continuity error): After the pool game is abandoned when Frasier accidentally jostles one of the players, the color and position of the balls on the table varies erratically between camera shots.
Goof (continuity error): During the scene in which Lilith is leaving Frasier's apartment and traps her handbag in the door, Frasier's shadow can be seen on the close-up shot of the door. During the shot, his shadow shows he is putting his hands in his pockets, but when the camera cuts back to Frasier, he has his hands by his sides.
Goof (continuity error): In Cafe Nervosa, when Niles spills the milk, the cups and towels on the table change between shots.
Goof (continuity error): The 'glockenspiel' clock is center of attention when it springs to life showing 11.45. A line later the clock is in shot again showing (apparently) 10.55 or so. When Frazier is admitted a few moments later it shows 11.20.
Goof (continuity error): Niles knocks over the milk jug in the café; he is thrown a tea towel to mop it up and he starts doing so. Seconds later the towel has disappeared and the table is dry. In the next shot the table is wet again and the towel has reappeared.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): Frasier makes references to a "French champagne" when there isn't any other kind. Only wine made from the grapes found in the Champagne region of France can be considered "Champagne". Everything is else is simply sparkling white wine.
Goof (continuity error): When Martin sits on the new vibrating chair Fraser bought,he comments at it being disgusting but earlier in the season his own chair had to be plugged in for it had a built-in vibration feature.
Goof (continuity error): The book which Frasier selects from the coffee shop library disappears then reappears between shots.
Goof (revealing mistake): The shots from exterior of the 'Winnebago' reveal it to be a studio mock-up. For example the parked windshield wipers make no contact with the windshield.
Goof (revealing mistake): The view from the hotel window is clearly a painted backdrop.