Factual error: There are many factors concerning Niles' post-operative state which are very inaccurate. After open heart surgery it takes at least three months for the chest bone to heal, Niles would not be able to drive a car for weeks, nor would he be able to lift or twist which he does on the show. Also, he doesn't sport a scar. Your chest bone begins very high up, just below your neck, his scar should have been visible on numerous occasions.
Suggested correction: He has a scar in season 11 episode 8. When he's naked in the cafe. A scar just like my dads from heart surgery.
While this mistake points out many different factors, I don't think this should qualify as a valid correction. The mistake is saying in season 10 he doesn't have a visible scar when he should and the correction points to seeing a scar a third of the way into season 11.
Continuity mistake: Marty is working on a jigsaw puzzle and its box is sitting on the edge of the table. During the scene it rotates ninety degrees. (00:03:50)
Continuity mistake: Roz is in her booth and Kenny is talking to her. Her hair is hanging loosely in front of her ear but as Kenny says, "To hash out some rules", her hair is suddenly tucked behind her ear, even though she never touched it. (00:06:05)
Continuity mistake: Roz is in her booth on the phone and the pencil on her clipboard changes position from shot to shot. (00:06:20)
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: When Frasier drives into and breaks the barrier at the parking garage, you can see that the barrier snaps very cleanly at an obviously pre-cut point.
Proxy Prexy - S10-E3
Continuity mistake: Frasier is talking about running for the condo board and lifts his coffee cup to his mouth. The shot goes to Martin who says, "You might be the best man for the job," and Frasier's cup is still on the table. (00:01:10)
Proxy Prexy - S10-E3
Continuity mistake: At the end of the scene where Roz is offered a melange by Daphne and Miles, and thinks they said a menage (a trois), the content of her fruit bowl changes repeatedly.
Continuity mistake: When Frasier and Roz are in Cafe Nervosa, and he is showing her his plan to trick Bulldog, Roz is looking at a magazine. She puts it down to look at his notebook but in the next shot she is still holding the magazine.
Continuity mistake: When Frasier is asking Noel for a favor, Frasier sets papers down on the desk which move from place to place and even disappear and then reappear between camera shots.
Rooms With a View (2) - S10-E8
Factual error: Niles' room is filled with flowers. Flowers are strictly forbidden for cardiology patients because of the high risk of infection. Niles being rich and all that would not change the fact that no flowers are permitted whatsoever - they could kill him.
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.
Don't Go Breaking My Heart (3) - S10-E9
Continuity mistake: Niles is at Frasier's apartment, visiting with Marty. He takes some crackers out of his black fanny pack and puts the fanny pack behind him. We see it there when he stands up but there is a shot where there is nothing on the couch behind him. It then goes back to showing the fanny pack.
Continuity mistake: This is a mistake within a mistake: Niles and Frazier are trying to get the wrapped presents out of F.'s apartment and replace them with others, to play a trick on their dad. They are getting completely mixed up, and presents are being taken out and then returned by mistake, mostly by Frasier. At one point Niles has a pile of gifts which he puts down on the table behind the couch - the shot shows him holding them, but the next shot shows a completely different pile of presents, even though it is supposed to be the same pile. Frasier picks this pile up. It soon changes again in the next shots.
Lilith Needs a Favor - S10-E13
Continuity mistake: In one shot Niles is on the phone with Daphne, in the next he's hung up the phone and put it away, without as much as a "goodbye".
Some Assembly Required - S10-E19
Visible crew/equipment: As Frasier and Roz are leaving the Grant's home you can see the shadow of a boom mic appear near the top of the door. It quickly comes into frame and you then see it disappear.
Some Assembly Required - S10-E19
Continuity mistake: When Frasier and Roz are at Cafe Nervosa together, just as Daphne enters, Roz's coffee cup goes from being held in her hand by her chest to down on the table in an instant.
Continuity mistake: Just after Niles has run out of Cafe Nervosa, the tip he left on the table keeps vanishing and reappearing between shots.
Continuity mistake: When Niles and Frasier are first talking to one another in Cafe Nervosa, Niles' coffee cup goes from held in his hand to sat on his saucer in an instant.
The Devil and Dr. Phil - S10-E21
Continuity mistake: When Bebe is talking to Frasier, she is eating a piece of celery. The piece changes size between shots, most noticeably at the very end of the conversation when it suddenly goes from large to tiny.