Critical Description
The FSNamesystem's startFileInternal allows overwriting of directories. That is, if you have a directory named /foo/bar and you try to write a file named /foo/bar, the file is written and the directory disappears.
This is most apparent for folks using libhdfs directly, as overwriting is always turned on. Therefore, if libhdfs applications do not check the existence of a directory first, then they will permit new files to destroy directories.