In the last three weekends, one of our directories on our accounting server (the one that happens to hold all the data for our "time and billing" software) has changed to read-only access for all users.
It only happens once a week, but I can't figure out exactly what time, but seems to be sometime on Saturday night. Event viewer has not show much to shed light on the issue. About the only thing I have scheduled is a thorough virus file scan at 6am Sunday morning. I've turned that off on that server, so will have to see if that is the issue.
Pretty odd though as the rights "look" right in Windows Explorer on the server, but if I go to the folder in question and go to the Advanced Rights window and select for inherited rights to be reset, the problem goes away.
Anyhow, something to monitor.