yep, NT had a history of service packs gone awry. SP5 broke more things than it fixed and SP6 broke one program specifically - Lotus Notes. which at the time was the dominant corporate email client. pressure from most major companies caused M$ to release 6a.

the interesting note to this story is how they broke notes. apparantly one of the Windows DLL files had a flaw in it. it was coded wrong, but it worked and affected nothing, so it was overlooked and left alone. well, notes used this DLL file and somehow worked because of the flaw in it. M$ being who they are, found this out and in an attempt to help get Outlook off the ground, fixed the flaw. which in turn stopped notes dead on thousands of Fortune 500 machines. not a good way to keep on the public's good side. Lotus blamed M$ for changing one of their files without notifying other large companies so there was no time to react, and companies freaked out. so SP6a was introduced with this one file put back to the orignal version.

and the point to all of this? wait a few months before you trust M$ because you never know what might break with an update.