Frasier tells Niles that he once saw Maris naked in the shower. Since Frasier also saw Daphne in the shower in Season Two's "Daphne's Room", that means he has seen both of Niles' wives naked in the shower. Of course, Niles sleeping with Lilith in Season Five's "Room Service" more-or-less makes up for it.
Niles: (lamenting an opera he's just attended) ...the soprano couldn't hit the E-flat above high-C to save her life.
An operatic soprano's vocal range is from C in the fourth octave (on a piano) to C in the sixth octave (high-C). If she could have hit the E-flat above high-C, she would have been classified as a sopranino, not a soprano.
When Martin asks Niles if he can have a romantic evening with Sherry at Niles' apartment, Martin says Sherry's apartment isn't an option because he's allergic to her cats. However, in "Dad Loves Sherry, the Boys Just Whine," Martin said he and Sherry were spending time at her apartment, and the only problem was that she lived in a fourth floor walk-up which was hard on his hip. He made no mention of her cats.
Niles: Just remember, always use coasters, no snacking in carpeted areas and close the art books after viewing so you don't damage the spine.
Fraiser: I believe they have the same rules in the Playboy Mansion.
Martin: Oh, hi there, Mrs Langer.
Ms. Langer: Ms.
Martin: Oh, right, Mis...(elevator doors close)...erable old cow!
Fraiser: You know this building has a grapevine Ernest and Julio Gallo would envy!
Pun on the word 'grapevine'. Ernest and Julio Gallo were the founders of the famous E & J Gallo Winery.
Second Title Card: WHAT WE HAVE HERE IS A FAILURE TO COMMUNICATE
This is the line made famous by "Captain" (Strother Martin) in the 1967 film Cool Hand Luke starring Paul Newman.
Another political thriller reference in this episode is Frasier's meeting in the parking garage with the doctor who wants him to run for condo board. This scene is very reminiscent of the scene in All the President's Men where Woodward and Bernstein meet with their mysterious informant Deep Throat.
Episode Title: Three Days of the Condo
The title is a reference to the 1975 thriller Three Days of the Condor.