Rest assured that all implementations of creating a directory share the property that if the directory already exists, nothing happens to it or its contents. (Different implementations might have different kinds of "failure" behaviors when this condition comes up, but they'll all leave an existing directory as-is.)

Now, if a directory contains a data file with a particular name, and you mistakenly do something that involves overwriting or replacing the contents of the file with something you don't really want -- e.g. rename($badfile,$goodfile); -- the damage will "succeed", of course.