Visible crew/equipment: When Frasier is standing through his sunroof to speak with the drivers stuck behind him in the parking garage, one shot shows Frasier's body and much of the rear window which features a very clear reflection of the boom mic.
Revealing mistake: About 1/3 through the episode, they are in Roz's apartment. If you watch when people enter or leave the apartment, the door has a handle lockset. It also has a lock cylinder about 1 1/2 feet above it on both sides of the door. The problem is, the side of the door has no drill hole where the latch assembly should be: it's just solid wood. The lock isn't attached to anything that would enable it to work.
terry sRevealing mistake: The guy who plays Clive in this episode has one of the most appalling accents I have heard. He's apparently from Manchester, but sounds like a sort of cockney South African. His accent slips around all the time; it's very poor. I wouldn't normally submit it, but his is so shockingly bad I doubt anyone would be fooled into thinking he was British.
David MercierContinuity mistake: When Frasier is talking to Claire in the store by the CD organizers you can see a wedding band on this finger. In the next shot it is not there and then it is back on again.
Morning Becomes Entertainment - S7-E19
Continuity mistake: In the initial scene where Frasier is talking to Daphne and Roz in Cafe Nervosa, Frasier can be seen wearing a wedding ring in certain shots. (00:01:10)
electricbiscuitYou Can Never Go Home Again - S3-E24
Continuity mistake: The flashback is set in 1993, but all the characters look the way they did three years later. Most noticeably, Frasier's hair is short and not long and wavy like in the first season. Daphne's appearance has changed quite a bit too.
littlestarThe Kid (2) - S5-E4
Continuity mistake: During the scene where Frasier and Roz are talking at her apartment, the mug that had been sitting on a tray moves off the tray and is sitting right on the table.
Continuity mistake: When Frasier and Poppy are sitting on his couch having a drink, the level in the wine bottle goes down and up and down throughout the scene, with no logical reason.
Sleeping with the Enemy - S3-E6
Character mistake: Noel Shempsky (the supposed Star Trek Nerd) attributes the quote "The needs of the many outweigh the needs of the few or the one" to James T. Kirk, when it was Spock who said it. (00:03:50)
SanguisContinuity mistake: Why can Lilith hear what they are saying through the door? In several other episodes, the characters have had arguments or insulted the person at the door without them being able to hear. Yet here suddenly it's possible for her to hear.
David MercierRoz, a Loan - S6-E9
Continuity mistake: Frasier and Niles are talking to Marty about the spa. As they speak, the magazines on the coffee table shift about 6 inches even though neither of them touch them.
Something About Dr. Mary - S7-E16
Continuity mistake: When we first see Daphne's wrist brace it is on her right arm. In the end credits it's now on her left arm.
Lummie
Continuity mistake: During the opening credits, Frasier is playing the piano and Martin is going through papers on the table. When Marty says "I don't know, Eddie, must you?" there is something pink lying on the table. The shot goes back to Eddie and then back to the table, but the pink item has disappeared.
Continuity mistake: There was an episode in which Martin attempts to end a feud between Frasier and Niles by telling a story about his detective partner. He starts with the line, "I never had a brother...", but in the episode about Frasier's cousin Nikos getting married, Martin's brother Walt was introduced.
To Thine Old Self Be True - S7-E20
Revealing mistake: When Reagan drops her bag in the hallway when she is talking to Frasier, a lettuce rolls towards him. It didn't fall from her bag: if you look closely you can see it was tossed into the frame from just outside the camera view, obviously to ensure that it would land at Frasier's feet for the comic effect of him kicking it back to her.
Purple_GirlSpace Quest - S1-E2
Continuity mistake: Frasier (and later the others) come back to his apartment to avoid the rain - they shake off umbrellas, and we even hear the storm at the very start of the scene. However there's no rain on the balcony - can't be covered, as we see in "Call Me Irresponsible" during the obligatory sex scene that when it's raining the balcony gets soaked.
Jon Sandys 
The Good Son - S1-E1
Continuity mistake: In the scene where Daphne first comes to the apartment, the beer can and remote control on Martin's side table change positions between shots.
pinkwaferThe Good Son - S1-E1
Continuity mistake: During the first episode of Frasier there is a painting of some vases in Frasier's apartment which is at the end of the hall leading on to Daphne's Room. In this episode the painting has a red and purple background, however in 'Here's Looking at You' (S1, 2 mins 52 secs) the exact same painting is now red and blue. It remains this new colour for the entire 11 seasons. (00:06:50)
Continuity mistake: When Frasier asks Martin "Where are your pants?", Marty is holding the ice cream carton underneath, but when the shot changes and he answers, "In the fridge", he is holding the carton by the side.
Continuity mistake: Near the start of the show, Roz is in her booth at KACL and takes out her lipstick and compact - we see her uncap the lipstick. Then Niles comes in an Frasier reintroduces them, and we see her uncap the lipstick again.
Suggested correction: First, not every single person from one area sounds the same. People can be influenced by a parent/other family members from different areas. I am native to the Southern region of the USA, but one of my parents is Northern. I have been told that I don't sound like a stereotypical Southerner. We don't know anything about Clive's family. Second, I have seen quite a few movies or TV episodes with actors whom I know to be actual British people, and Clive doesn't sound much different than any of them.
I am English and believe me that accent is the worst example of English I have heard in my life.