In this episode, Frasier (Kelsey Grammer) says he doesn't speak German, and they have to converse with the fencing instructor in Spanish, with Maris's housekeeper translating. However, Frasier was able to speak German in both an episode of Cheers (1982), and a later episode of Frasier.
In the credits, Niles (David Hyde Pierce) gives Maris a rose when she is in the tank, but a few episodes earlier, Niles had a fight with Maris and he said he couldn't buy her roses because she was allergic.
When the vase is broken during the sword fight, notice the way it has been damaged. When Niles (David Hyde Pierce) comes back down the stairs, the damage to the vase is different.
Martin talks to Niles about the affair between Hester and a family friend. However previously he told Niles and Frasier that Martin had cheated on their mother not the other way around. When Frasier found out the truth Martin told him not to tell Niles.
As he's exiting Frasiers apartment Niles says "I'm pretty sure I just swallowed a whole twist of lemon", however in several prior shots you see no lemon the glass.
When Niles says he went only to the 'Or-eh-gone' border because of the fresh fruit not being allowed past it, he was wrong (likely due to the show being taped in CA). In reality it is the California border which holds back fresh fruit when one enters from OR or elsewhere. This is due to the enormous farming industry in CA and the danger of introducing foraging insects into it, which could wreak much havoc to the plants, let alone the financial repercussions for damaged plants/crops. Oregon and Washington don't have the fresh fruit restrictions, normally.
Niles says that he was stopped at the Oregon border check. There IS no Oregon border check between Washington and Oregon.
When Frasier reminisces about the "Visible Man and Woman" models their mother bought Niles for Christmas, he says, "He [Niles] had to glue all of the internal organs in the right place." Those models--manufactured by the Renwal Model Company--did feature the body's internal organs, but they were not meant to be glued in place. They were designed to be removable, so one could see how each organ aligned with the rest.
"Gunnar" is not a German name; its German equivalent would be "Günther."
The episode ends with Frasier and Niles seated side by side at Niles' piano playing the opening of Mozart's Sonata in D Major, K.448. In fact this is a piece for two pianos, not piano duet, which means Frasier and Niles should be at separate pianos.
During his tussle with Gunnar, Niles leaps onto the hearth of his light fireplace and places his hand on the underside (i.e. fire-facing) surface of the fireplace's metal surround, keeping it there for several seconds without the certain severe burns that should have resulted.
When Frasier addresses the sensory deprivation booth and speaks of Maris' affair, he mistakenly thinks that Maris is inside. When he opens the booth, he discovers Niles instead, looking sad and forlorn from learning of his wife's transgression. But being inside a sensory deprivation booth, Niles would not have been able to have heard anything Frasier was saying.
When Marta confuses "Mr" and "Mrs," Niles tells Frasier that Marta has problems with her pronouns. "Mr" and "Mrs," however, are not pronouns, they are honorifics.
Several times Frasier says 'translate' when he should be using the word 'interpret'. Written word is translated while spoken word is interpreted.