Factual error: Simon, having just been kicked out of Frasier's apartment, is trying to convince Roz to let him stay at hers. In the background the set shows a wharf area with at least one tall ship, Alito's Restaurant, Pier 45 Shed A, and Fisherman's Grotto. These are all located at Fisherman's Wharf in San Francisco, not Seattle.
Character mistake: When Kenny's wife walks through the door, Frasier says "Aw Janet come on" instead of calling her Janice. (00:11:41)
Goodnight, Seattle: Part 2 - S11-E24
Character mistake: Frasier says, "it's only an hour flight to San Francisco..." Seattle to San Fransisco is two hours minimum.
Continuity mistake: At the end of 3-10 in the airport with Kate, Frasier says that he has an allergic reaction to cats but here he cat sits for Mr. Bottomsly.
Suggested correction: This has been addressed in the Questions section. Frasier is apparently choosing to deal with his allergies, in order help the cat's owner. It's also possible for allergies to be triggered by one cat, but not another. It depends on the amount of dander that the particular cat produces, and the level of Fel D. 1, a protein in their saliva.
Continuity mistake: When Niles and Frasier are in the cafe discussing how many patients have fallen in love with them, Frasier claims 13. But in the episode "The Maris Counsellor," he states no patient has ever fallen in love with him. OK, time has passed, but he's done hardly any private practice since then, and any he did do was shown anyway.
Something Borrowed, Someone Blue (1) - S7-E23
Visible crew/equipment: As Roz and Simon are leaving, and Niles enters the apartment just before telling Martin he's married, the camera pans far too wide. On the left-hand side of the screen, you can easily see the end of the set and crew members in jeans standing around.
Visible crew/equipment: A boom mic is briefly visible when Frasier is talking with Kate in the hall. (00:11:09)
Revealing mistake: Towards the end of the episode, as Frasier is on the lounger by the windows, a power cord is on the ground going through the far left window (where glass should be), possibly going to the lamp.
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)
Revealing mistake: When Frasier and his guest are standing up to leave, the girl behind the counter is watching them for her cue to come over.
Revealing mistake: Even though she has customers in front of her, the barista can be seen watching Frasier and his guest as they are talking, so she can see her cue to come over.
Factual error: When the barista hands an umbrella to Frasier's guest we can see she is wearing platform flip-flops. Health code in Seattle forbids open-toed footwear for employees in food service establishments, which would include a coffeehouse.
Boo! (a.k.a. I'm With Her) - S11-E16
Continuity mistake: In the first scene after Niles walks in and takes his coat off. The left-hand side of his shirt collar alternates between tucked underneath his jacket and sitting outside it.
You Can Never Go Home Again - S3-E24
Continuity mistake: When Frasier turns up to the office on his first day, having never met Roz before, he has the briefcase that Roz got for him as mentioned by Roz and Niles 'Fool Me Once, Shame on You, Fool Me Twice...'. How can this be if they've never met before?
Continuity mistake: Marty throws his coat on the stairs, but it moves to the landing.
The Show Where Lilith Comes Back - S1-E16
Visible crew/equipment: Halfway through the episode the boom mic is visible at the top left of the screen above Lilith. (00:10:00)
Rooms With a View (2) - S10-E8
Character mistake: In the x-ray on the right, the x-ray is flipped. The heart should be displaying on the right.
Visible crew/equipment: As Frasier walks into the kitchen near the start you can see a crew member with arms folded at the side of the table on the other side of the lamp. (00:03:55)
Continuity mistake: Frasier is opening a bottle in the kitchen while talking to his father. He takes the cap off the same blue bottle twice. (00:05:07)
To Kill a Talking Bird - S4-E14
Visible crew/equipment: After Frasier starts the fireplace and it scares Niles' bird, you can see the hands of a bird handler by the stairs on the upper right of the screen. (00:12:21)