Ask HN: What broke on 29th due to a leap day?

I was wondering who got today a lesson that is taught every 4 years.

Today my company's timesheet web app was broken and I don't think it is a coincidence.

I also remember when I was in a mobile app project with a nice scrolling calendar in times around iPhone 4. Our manual tester saw that something strange is happening around February. We couldn't understand why it was so junky around a specific date. After a too long day of debugging I learned my lesson while moving everything to Objective C date/time API.