Hey, It's That Voice!: J. Peterman's actor can unmistakably be heard as the voiceover in current Coors Light commercials.
Old Shame: Regis Philbin loves the series, so he was disappointed that when he guest starred, he was given a line that he didn't think was funny (three variants of "This guy's bonkos!"). The producers insisted it would be hilarious, but his lines didn't get very many laughs with the studio audience. To this day, Regis dreads watching that episode for that reason.
The Other Darrin: Phil Bruns originally played Morty Seinfeld, but after his initial appearance, he was played by Barney Martin.
Larry David originally played Newman in "The Revenge" (he provided his off-screen voice), but his first on-screen appearance was by Wayne Knight. In syndication, Knight dubbed over David's voice in "The Revenge" to provide some continuity. However, both versions of the episode are available on the DVD set.
John Randolph originally played Frank Costanza in "The Handicap Spot". Later, Jerry Stiller replaced him in "The Puffy Shirt". For syndication, John Randolph's scenes in "The Handicap Spot" were re-shot with Jerry Stiller in his place, to provide continuity. However, as with the previous example, both versions are available on DVD.
Lloyd Braun was initially played by Peter Keleghan, and later played by Matt McCoy in "The Gum" and "The Serenity Now".
Vic Mackey met Kramer in Jerry's apartment before joining the LAPD.
Spence Olchin worked in a video store before selling subway tokens.
What Could Have Been: Larry Miller was originally cast as George, but it was decided that, just to make George a little bit more pathetic, he needed to be short. Miller eventually appeared as a doorman that could give The Janitor a run for his money.
The project began as just a 90 minute special about a day in Jerry's life, and how it inspires his stand-up material that night. Jerry and Larry David couldn't quite stretch the script to fill the 90 minutes, so they reduced it down to a half hour as a series pilot. As Jerry would later say, "We couldn't make 90 minutes, so we made 90 hours."
Elaine's father was intended to be a recurring character, but then they hired the notoriously ill-tempered and intimidating Laurence Tierney to play him, and after filming the episode, no one wanted to work with him again. Among other things, Jason Alexander and Jerry were genuinely intimidated, and Tierney was seen stealing knives from the apartment set.
The waitress in the pilot was meant to be a recurring character, but the producers thought she was too abrasive and the show needed a female lead, so Elaine was written in.
An episode called "The Bet" was scripted but never produced. When the script, whose plot revolved around Elaine buying a gun, reached the table read stage, it came off as unfunny and unnecessarily dark. Pretty much everyone present realized on the spot that the concept wouldn't work, so the entire script was shelved.
"The Bet" lives on as an Internet urban legend in the form of a "creepypasta" story alleging the episode was filmed and all but one copy destroyed. Naturally, the episode supposedly contains disturbing and supernatural phenomena happening to the actors and crew.