SQL Server 2008 Authentication mode changed on its own This morning we experienced an issue where our production server seemed to decide to change from Mixed authentication mode to Windows only authentication of its own volition. I was curious to know if any of you other DBAs out there had ever experienced this or might be able to point me in a good direction to try and make sure this doesn't happen to us again. The only other thing that seemed to be going on database wise, beyond typical use, was that the transaction log had just completed a backup.
The chances that the Authentication Mode changed on it's own is pretty near zero. More likely, it got changed by accident.

This is a change that requires a restart of the SQL Server service in order for it to take effect. What likely happened is that the Authentication Mode was changed sometime since the last restart (which could have been months ago)--then, when the instance restarted, the change "suddenly" took effect seemingly on it's own. 

This particular configuration is stored in the registry, so if you have OS backups of your server, you could do some research to go back in time to try to determine when it changed.