Continuity: Early in the show, Niles has his hand up on his tote bag talking about the mosquitoes at the cabin, but the last shot shows him with his arms down by his sides, with no time for the change.
Great sites
Quotes
[Frasier tries to convince Niles that sleeping with Nanny G is the right thing to do.]
Niles: Did you say something? Your penis was talking so loud I couldn't hear!
Trivia
Frasier was not originally meant to have a brother. But the show's creators saw David Hyde Pierce and realised he looked remarkably like a younger Kelsey Grammer. So the character of Niles was created especially for him. See more...
Frasier (1993) - 3 mistakes in "Momma Mia"
starring David Hyde Pierce, Jane Leeves, John Mahoney, Kelsey Grammer, Peri Gilpin (add more)
Updates: 
Series 4
Momma Mia (season 7, episode 1)
Continuity: In the kitchen of the cabin, Niles throws his blue napkin on the table, which had nothing on it. A couple of shots later, as he and Frasier leave the kitchen, the table now somehow has 3 blue napkins on it.
Continuity: Marty brings a six-pack of beer to the cabin, and puts it down on the table where it stays for the scene - but during the scene one of the cans is sticking out for a few shots, then returns to normal later. No one went near the beer during that time.
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