Leap year exposes bugs in software.
Leap year is a special test condition that is often included in acceptance testing. However, methods of simulating different dates have unexpected holes in them. Computers have a *lot* of OS software running to support one's program. And there are lots of unexpected ways that the OS (and other supporting) software can obtain dates and process dates. That means that even when you tested it, something might turn up.
Also means that the fixes to your store's software won't be tested thoroughly until the next leap year.