Chekhov's Sneeze

Janet tells her friend Steve that she's reduced her boyfriend standards from a list of positive traits to just one; a guy who will "bless" her when she sneezes. When her current boyfriend Cliff fails to do so, she breaks up with him and resists his efforts to win her back. When he "blesses" her at the end she realizes that he has changed, and they get back together.

Example of:
Chekhov's Gun