This is not a bug, but perhaps the documentation could further clarify that the set of valid values for the mode parameter is platform dependent. The [POSIX specification][1] for mkdir() states that "[w]hen bits in mode other than the file permission bits are set, the meaning of these additional bits is implementation-defined". On Linux, mkdir honors the sticky bit (S_ISVTX). On OS X, on the other hand, the [man page][2] states that "the behavior of mkdir() is undefined when mode bits other than the low 9 bits are used. Use chmod(2) after mkdir() to explicitly set the other bits".

1. http://pubs.opengroup.org/onlinepubs/9699919799/functions/mkdir.html
2. https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/mkdir.2.html