On Fedora 18 the startup script times out and then kills mongod because it can't read the pid file.

Apr 01 10:35:00 gunnarr.bn.dev mongod[9596]: Starting mongod: about to fork child process, waiting until server is ready for connections.
Apr 01 10:35:00 gunnarr.bn.dev mongod[9596]: forked process: 9605
Apr 01 10:35:00 gunnarr.bn.dev mongod[9596]: all output going to: /var/log/mongo/mongod.log
Apr 01 10:35:01 gunnarr.bn.dev mongod[9596]: child process started successfully, parent exiting
Apr 01 10:35:01 gunnarr.bn.dev runuser[9601]: pam_unix(runuser:session): session closed for user mongod
Apr 01 10:35:01 gunnarr.bn.dev mongod[9596]: [ OK ]
Apr 01 10:35:01 gunnarr.bn.dev systemd[1]: PID file /var/run/mongo/mongod.pid not readable (yet?) after start.
Apr 01 10:40:00 gunnarr.bn.dev systemd[1]: mongod.service operation timed out. Terminating.
Apr 01 10:40:00 gunnarr.bn.dev systemd[1]: Failed to start SYSV: Mongo is a scalable, document-oriented database..
Apr 01 10:40:00 gunnarr.bn.dev systemd[1]: Unit mongod.service entered failed state

The default configuration has the pid file written to /var/run/mongodb/mongod.pid, but the init.d script (and systemd) looks for it to appear in /var/run/mongo/mongod.pid