I've run into the situation a few times with OpenVPN, mainly when I'm "fiddling" with my configurations.
I think the scenario occurs when a client is in the middle of establishing a link and I try to pull the server side down.
The server instance tries to stay alive and complete the link so the restart ends up failing (sometimes "silently").
Normally a manual command line kill of the session solves the issue. Worst case you're stuck with a reboot (very rare).
Once you stop playing with the config files on both ends (especially mid-connect), I've found OpenVPN to be very stable.