I did some looking around for the ending of the infamous Breakfast Club joke. For the record, the joke goes like this:

A naked lady walks into a bar with a poodle under one arm, and a two foot salami under the other. She leaves the poodle on the table. The bartender says, "I suppose you won't be needing a drink." The lady says "Why do you say that?" and the bartender says...

At which point John Bender (played by Judd Nelson) falls through a roof. However, even though there have been several competitions and the like, the official punchline has never been found. One fansite theorizes that the ending is just "OH SHIT!" which is what Bender yells as he falls, although it doesn't make sense in the context of the joke.

According to people in the know, the joke was ad-libbed by Judd Nelson on the set. In fact, he only had a punchline, "I forgot my pencil," and was trying to make up a joke to fit. Judge for yourself whether you think he succeeded.

Audited April 30, 2002. Sources: http://www.internetdatingbook.com/tbc/ and http://www.riverblue.com/hughes/trivia2.html