Governments are free to change timezone rules at will with no real advance notice. Case-in-point, tzdata2012b was just published a few days ago and already on the tz mailing list Morocco's DST date is changing and Haiti apparently changed as well.

The problem is exacerbated by the fact that most systems rely on OS tzdata instead of shipping it themselves, yet in many places the administration of the OS is separated / walled off from the devs who maintain the apps which actually suffer from the bugs.

I feel bad for users who live in countries whose governments play with DST law on a whim, because they are certainly destined to encounter bugs throughout the Internet because it isn't always reasonable to ask all administrators around the globe to patch all production systems in a matter of a few days.