I encountered this when working with Search and Insert cursors. I had a script which was working fine, all tables being accessed and written to within a File Geodatabase. I made some script changes, in which tables were now being written into a simple directory folder. Absent mindedly, I initially did not change table names explicitly with ".dbf" in the script, so within the folder they were written as default INFO tables, and the problem surfaced. I created a new FGDB in which to write the tables as default dBASE and the problem disappeared. As per the above response, properly deleting cursor resources is also probably important.
-Ryan