Ha. I had an intern who wrote code that blew up if the timestamp of our DB server was ever >= 1 second after the timestamp of our app server.
It couldn't fail at his desk, because he ran both servers locally while developing. And it wouldn't fail in production in the morning, since we had a cron job that synced the clocks at midnight. It took until after 5pm for the clocks to drift enough, meaning I was on call and the intern was not.
But the bug didn't crop up until his code had been in production for a couple of weeks, so it was a real pain in the neck to track down.
It couldn't fail at his desk, because he ran both servers locally while developing. And it wouldn't fail in production in the morning, since we had a cron job that synced the clocks at midnight. It took until after 5pm for the clocks to drift enough, meaning I was on call and the intern was not.
But the bug didn't crop up until his code had been in production for a couple of weeks, so it was a real pain in the neck to track down.