I installed Zen Cart on one of my servers for a client to test the software as he intended to use it if it fit his needs. It appeared to be a good solution for that client but overnight some how through the Zen Cart software the MySQL database user file was accessed and all users were deleted and the user folder on the server was corrupted and unable to be repaired.
I found the problem through a secondary server that monitors all access to the servers through the network and found exactly what was done and how. The person who did this did a very good job at covering his tracks and I would have never found out what happened if I didn't have a secondary server running the log copies and such.
What I had to do is log into the server as a root user and upload a copy of the user records on to the server and reset the file permissions as it is a Linux server. Once that was completed all users were able to access their MySQL databases once again with out any problems and no data was lost in any of the databases running on the server.