Baker's Dozen
A regular dozen is twelve, but a baker's dozen is 13. Years ago in England there were strict penalties for those who gave short weight. During this era Bakers were usually not educated and terrible counters. So to avoid any penalties, the bakers would give 13 instead of 12 or a dozen, just in case they miscalculated.

This from a website visitor:
The Baker's Dozen comes from Colonial America where bakers were hung for making the loaf light. That is why they give 13 instead of 12. Additionally, they decided to use the Avoirdupois pound instead of the Apothecary. The Avoirdupois weight is the French standard of dry measure. All of our dry measures in America today (tsp, tbsp, cup, etc.) are Apothecary except the pound which is still Avoirdupois. The Avoirdupois pound is 30 grams heavier than the Apothecary measure.

This from another website visitor:
I think this actually originated from the fact that even though you follow the same exact recipe, two batches can turn out differently (weather, humidity, etc.) So to be sure that the batch is ok, the baker adds one extra to be tasted when the baking is complete - leaving a full dozen to sell (来源：英语博客 http://space.englishcn.com)