Season 3, Episode 8
"The Last Time I Saw Maris"
Context: Niles' wife, Maris, has disappeared and has been missing for days. Niles is understandably concerned, although it turns out Maris has simply been on a shopping spree in New York. At his brother Frasier's urging, Niles stands up to Maris, who in turn demands a divorce and kicks Niles out of the house. In the aftermath, Frasier takes his brother into his home, and it is the following morning when Frasier is in his kitchen trying to find some food for his father's dog, Eddie. Luckily, one of his housekeeper's dishes isn't fit for humans...
Exegesis: This may be the only instance when "humans" is used as a sound-alike to "Mormons". We've had "moron" several times, "hormones", too. But humans? It's a stretch, but the end result is amusing.
Of course, Mormons can eat shepherd's pie, but it's interesting to think there's some kind of food that is fine for everyone else but not "fit" for us... It makes us sound kind of important... This reference is basically a variation on a comic staple, where someone says something privately or under one's breath, and is overheard by someone else. When asked to repeat or clarify, the second person repeats instead a (usually implausible) sound-alike version of their original line. It's the dichotomy between what was actually said, and the hastily assembled alternative that produces the comic effect. Sometimes the whole phrase is replaced, sometimes just one word, and typically the word change renders the original phrase nonsensical or otherwise amusing, as was the case in this example.
Frasier was a great show - a fine example of classic farce on a weekly basis. This isn't the only time Mormons came up in the series, either; Frasier hires a Mormon agent (and former MoTab singer) in the Season 5 episode "The Zoo Story".