In Office, when you work on a document on a "windows share", a lock is put. This is the same approach in Visual Source Safe, exclusive locking. Most of the time, it's completely useless, because there is someone in charge for the file edition (but the Office programs don't let you view a document in a simple way). If there are several people in charge of some piece of work, for God's sake, don't use a format which is not externally diff'able. So, locks are put... and they are removed when all goes well, that is, most of the times. And when something happens:

• Word goes nuts (just try Word 2002 on Windows XP, you have a 2 hours TTL)
• a "windows share" unproper disconnection
• windows (from W2K, windows isn't less buggy, but bugs don't lead systematically to a GPF anymore)

the lock is lost, and your stupid Office program is proud to announce that the document is locked by yourself when you try to reopen it. The solution? Reboot 3 times. Have fun.