There is a race condition in file mkdir with two process atempting
to create the same multi-level directory path at the same time will
cause one of the processes to get an EEXIST error. This is caused
by one process creating a directory between the time the other
process checks if the directory exists and attempts to create it.
While this is uncommon on a single system, it's easy to reproduce
on a large cluster with a shared file system.