http://bugs.python.org/file19849/mkdirs.tr.diff introduced a patch with this code in it:

+def _get_masked_mode(mode):
+    mask = umask(0)
+    umask(mask)
+    return mode & ~mask

This changes the umask of the entire process.  If another thread manages to create a file between those two calls then it will be world read/writable, regardless of the original umask of the process.

This is not theoretical: I discovered this bug by observing it happen.