(Cross-posted at Jennifer's History and Stuff.)
The number 13 is thought to be an unlucky number, with some people refusing to live in a house numbered 13 or stay on a hotel floor numbered 13.
Well, around 1700, the belief that having 13 people at a meal was unlucky was fairly widespread. Therefore, the number 13 must be unlucky because of the Last Supper, right? After all, 13 people participated in that, and one was dead soon thereafter.
But actually, early Christians loved to emulate J.C., and that included organizing themselves into groups of 13. You'd have 12 nuns and their superior, for example, making a group of 13. The number 13 was awesome!
So what happened? A little thing called the Protestant Reformation, actually. During the Reformation, the obsession with groupings of 13 was itself called "superstitious", and the practice was forbidden.
This led to the number 13 immediately having a "bad" connotation that turned into unlucky over the years...becoming its own superstition!
Reference: Most of the material from this post was found in Steve Roud's The Penguin guide to the superstitions of Britain and Ireland.