Bugs in time-based code are great. The most amusing example I've seen was an intermittently-failing automated test case which, I discovered after careful scrutiny of the results database, failed only on Sundays.

There's also the classic time zone unit test "convert this Europe/London timestamp to America/New_York time, verify that it is now 5 hours behind", which, due to Bush's pointless Daylight Saving shift, now fails for two weeks twice a year. Of course, two weeks is just short enough and the bug is just inconsequential enough (the time zone code itself is working correctly, after all, it's the test case that's broken) that the regression goes away before anybody fixes it.

I can only imagine how much otherwise robust code flips its lid when presented with, for example, a leap second. It's the number one argument for the use of mock objects in testing.