This happens when I create courses and also backup and restore individual courses. I get a random amount of resource files from random other courses appear in the new course. e.g. powerpoint, word, excel or pdf. These files are copies of files in other Moodle courses.
Just to keep the random nature of it all going, it does not happen all the time.
I am using 1.6.1+ on an IIS server.
Mike McDermott