Elaine and her old boss find that selling just the tops of muffins is more profitable than selling the whole muffin (which consists of the top + the stump). So are the top and the stump complements or substitutes? Neither--the stumps are an economic bad, which reduce utility. Evidence for this is found in the fact that homeless people won't eat the stumps that the muffin-top restaurant throws away, unless they come with the tops as compensation.
Jerry is dating a girl but really wants to date her roommate. George suggests that the only way to make the switch is to propose a menage a trois to his current girlfriend, which will turn her off and her roommate on. Jerry follows through on George's plan, and finds that both girls are "into it." But Jerry can't follow through—and George can't believe it. To Jerry, the roommates are substitutes; to George, they are complements.
Sheldon and Leonard are on a team against Amy and Penny in a game of Pictionary. During one round, Leonard draws what looks like a chocolate chip cookie on the board. Sheldon guesses incorrectly several times, and the ladies win the round when Penny guesses correctly. Leonard argues that Sheldon should have guessed correctly, and in response, Sheldon fires the criticism back at Leonard.