Stacy Keach plays a Manipulative Bastard, politically incorrect older man who seems to love drinking, much like his role on Titus.
Kyra Sedgewick plays a Deputy Chief sent to evaluate the 99, just like she played Deputy Chief Johnson in The Closer. She even has a rivalry with another straight-laced cop. At least she's using her actual accent this time.
Luis Moncada plays Tito Ruiz, the Giggle Pig kingpin, and is wearing the exact same suit he wore on Breaking Bad where he played Marco Salamanca, another menacing guy involved in the drug trade. As a bonus, Jake is very obviously dressed up like Jesse Pinkman in the same scene.
In "The Apartment", Gina and Jake reference being friends since they were kids and hanging out together. Andy Samberg and Chelsea Peretti, who play Jake and Gina, have actually known each other since they were kids.
Terry is a talented artist. Terry Crews originally went to university on an art scholarship before earning a spot on his school's football team and going onto the NFL.
Celebrity Paradox: Saturday Night Live, which had B99 star Andy Samberg on it for several years, apparently exists in this world, and Peralta even interacts with Adam Sandler...who apparently doesn't notice how much the detective resembles his That's My Boy co-star.
Dawson Casting: Inverted; while his age has never been specified Captain Holt is old enough to have been an active (albeit fairly young) detective in 1979. Andre Braugher, the actor who plays him, would have been 17 at the time.