When Sheldon learns that the neighbors above their apartment will move out, he begins to list all of the negative externalities that they didn't produce.

Amy talks about tampons which upsets the guys.

Howard and Sheldon feud over a parking spot.

A new part of the Roommate Agreement includes payments for all of Penny's bad habits.

A Kenny Rogers Roaster restaurant opens across the street from Kramer. He can't stand the red glare from Kenny's neon sign, and moves into Jerry's apartment. But he becomes hooked on Kenny's chicken, and eventually accepts the red glare in exchange for access to the chicken. When Kenny's shuts down, the lights go out, and Kramer's overall welfare falls—the benefits of the chicken outweighed the cost of the glare.