Ursula Buffay (Lisa Kudrow), the waitress, is the twin sister of Phoebe Buffay (Lisa Kudrow) from Friends (1994). In one episode of Friends (1994), Fran (Leila Kenzle) and Jamie (Helen Hunt) visit the coffeehouse and mistake Phoebe for Ursula.
While Paul Reiser had the same actors playing his parents for the entire run of the series, Helen Hunt's Jamie had three pairs of actors playing Theresa and Gus Stemple; Nancy Dussault and Paul Dooley; Penny Fuller and John Karlen; and finally Carol Burnett and Carroll O' Connor.
In one episode, a character asks if they saw the movie about those aliens that burst from people's chests (the Alien movies), and Paul Reiser says, "Only the first one." Reiser starred (and gets killed) in Aliens (1986).
In one episode, Paul and Jamie see Ursula after a long time of absence. When they ask her where she's been, she says, "I've been hanging out with friends". Ursula is portrayed by Lisa Kudrow, who starred in Friends (1994), thus her absence in this show.
Steve Buscemi appears in an episode in the first season, which was inspired by an actual incident between Paul Reiser and him. Before Buscemi began acting, he tried his hand at doing stand-up. One night, he bombed so badly, that another comic had to take his place. That comic was Reiser, and the episode pokes fun at the incident.