Worked on a project where devices just magically froze, but only during the month of February!

Turned out the people who had written the firmware had decided to do their own time math to save space and had put in an exception in the code for leap year values. Except instead of February 29th, it kicked in for the whole month. And the math was wrong so you ended up with negative values.

The product was due for launch in March of that year and was headed to manufacturing. It was by sheer luck that someone ran a test on February 1st and caught the problem.

Don't mess with time in code, kids.