This is the first episode we ever see Mr. Sheffield's bedroom.
In the opening scene in the livingroom when Mr. Sheffield complains that his singer dropped out, the resume he is reading belongs to Kurt Jacobs, Fran's ex-boyfriend from "A Fine Friendship. "
The title is a reference to the old phrase "What The Butler Saw" concerning voyeurism, specifically a butler peeping at the ladies of the house through the keyholes of their bedroom doors. It is frequently suggested in the series that Niles knows everything that is going on in the mansion because he eavesdrops on the family while "polishing" the keyholes.
While most British singing acts who have a strong English accent cope an American accent when singing (such as Sarah Brightman) and it has been claimed that it's impossible to maintain any British accent when singing, here Daniel Davis (an American) sings with a strong British-English accent.
This is the only episode that we see Nadine, Fran's sister, appear in.