Did you ever notice how in the last season, there were so many self referencing lines?. At least they weren't as horrendous as Roseanne, and most of them were funny, but at least Seinfeld didn't get too full of himself.
Butter Shave, Jerry: "He's a time-slot hit." (In reference to the fact there were so many new NBC sitcoms launched in the 8:30 timeslot after Seinfeld so that viewers would just keep watching.)
Serenity Now, J: "There's more to life than making shallow, fairly obvious observations." (A dig at the Seinfeld observational humor rap)
Frogger, Jerry: "You sure have a lot of friends. How come I never see any of these people?" (A reference to the fact nobody has ever seen any of Kramer's friends)
Bookstore, Elaine: "We made out at the table like our plane was going down." A finale pre-reference.
Burning, Jerry: "Showmanship George. When you hit that high note, say goodnight and walk off." Referencing the real-life reason Seinfeld ended the series, to "leave on a high note, and always leave them wanting more."