Reported here originally:

https://groups.google.com/forum/?fromgroups#!topic/mongodb-user/7o_bYPUJuSU

This has been recreated with and without authentication. The keyfile/auth version of the bug is much more painful because there is no way to remove the lock and get out of the bad state without sending kill -9 to mongod. With a non-authenticated set up it is possible to issue the unlock command.

Repro steps below.