We recently ran into a problem with check_dir_exists() and having it recursively create directories on Windows. Example call:

$CFG->dataroot = 'C:\xampp/moodledata';
check_dir_exists('C:\xampp/moodledata/path/to/dir', true, true);

What is passed to mkdir:

C:\xampp/moodledata/C:/
C:\xampp/moodledata/C:/xampp/
C:\xampp/moodledata/C:/xampp/moodledata/
C:\xampp/moodledata/C:/xampp/moodledata/path/
C:\xampp/moodledata/C:/xampp/moodledata/path/to/
C:\xampp/moodledata/C:/xampp/moodledata/path/to/dir/

Now, same call with the attached patch:

C:\xampp/moodledata/path/
C:\xampp/moodledata/path/to/
C:\xampp/moodledata/path/to/dir/

Since this method is called everywhere, I don't see how this problem hasn't come up before, which leads me to believe that I'm missing something. I did not experience the problem first hand as I develop on a Mac but as you can see from my simple test above, it is not working as intended.