Aha, got it -- while removing /a/b/c/d, there's no easy way to detect
that b or c has become a symlink.

I.e.

given directory tree

a
`-- b
    |-- c
    `-- d

1. os.rmdir('/a/b/c') succeeds
2. execution is suspended
3. '/a/b' is made a symlink to a path that contains 'd'
4. '/a/b/d' is neither a symlink, nor has it's inode been recorded, so
os.rmdir('/a/b/d') succeeds

I'm afraid the solution for the Perl bug is susceptible to the same problem.