On December 31st, all 30GB Zune users woke up to their music players not working. In a rarity for Microsoft problems, the source for for the problem was found. There is a good explanation of the problem here.
There are two lessons to be learned from this: 1) be careful of your looping conditions. 2) Try and write your code in small snippets that are testable, and write tests! A simple iteration through the total amount of days including a leap year would have caught this bug.
Just FYI, I’m not saying I would have been good enough to catch this, but it is worth writing down so I try to remember for myself.