A baker's dozen

A dozen == 12
A baker's dozen == 13

Why?

It is believed that this came from medieval times, when bread was sold by weight. If bakers were found to have sold "underweight" bread, they would be punished. So when people buy a dozen, they would give another loaf just in case.

Another theory is that bakers would charge vendors 13 loaves for the price of 12, making it something like a wholesale price, so the vendors can make a profit on their side.