If there are two directories with the same path but in different cases, get_directory_files will return files from both on MySQL. To replicate:

• Create a folder resource
• Add two directories with the same name but in different cases (e.g. "lab_data" and "Lab_Data")
• Add a file to one of them
• Browse to the other
• You should see the file

(I think this may also be the root cause of the issues described in http://moodle.org/mod/forum/discuss.php?d=173393 and reported in MDL-29225.)

This is caused by the fact that MySQL does case-insensitive string comparisons by default (http://dev.mysql.com/doc/refman/5.1/en/case-sensitivity.html). There seem to be two possible fixes for it, neither of which look as if they'd be easy to implement:

• Change the collation of the mdl_files table
• Insert the keyword BINARY into the SQL before the SUBSTR clause