‘Baker’s dozen’
The phrase “baker’s dozen” refers to 13 rather than 12 and dates back to medieval England. When selling a dozen loaves, it was common practice for bakers in that time period to include an extra one in order to avoid being accused of shorting customers.
This was due to a law passed by King Henry III that standardized the weight of a loaf of bread, after which bakers could get in serious trouble for giving a customer less than what was paid for.