by Ken Task.
Just thought of a config that might exhibit that behavior.
2 sites ... Moodle A, Moodle B
Moodle A installed first and used the 'defaults' for the database setup ... db name moodle **prefix mdl_**
Moodle B installed second or later and attempted to use the defaults for database setup but may have found issues in installing. As a 'work around', installer used the same db name (moodle) but a different prefix ... mdlB_
The same DB user and DB password could have been used in both instances. And that would not cause any changing of data in tables by itself ... it's just an authorization thing and what priveleges - alter, etc.
So ... don't have sites/DB's set up that way to test things ... but if you have some sort of tool to work with DB's (think phpmyadmin could do this for sure), select a database ... then show tables.
Show tables should have/show the prefix in front of the table names.
'spirit of sharing', Ken