I have noticed a lot of these moments in the early series, where you can often find Frasier giving his disapproving look to Niles.
One moment I heard today really kept me chuckling.
The episode is Roz's Turn, Roz wants to do a recording of a playful girlfriend and her uninterested partner, and asks Niles if he would like to be Daphne's boyfriend, and Daphne decides to be a bit naughty with what she says. She mentions wearing nothing but an apron and high heels, and wearing a 'whipped cream teddy' on the bed or in bed. Roz asks Niles if he is uninterested, and after a long pause, presumably to think of Daphne in those scenarios, he says he is uninterested.
Then later, Niles replays the recording where Daphne mentions the whipped cream teddy, and realising people are present, he stops the recording and apologises. Classic!
What are your favourite moments?