Running out of ionotify watches will result in a "No space left on device" error when stopping or starting a service. The relevant function is service_watch_pid_file here: http://cgit.freedesktop.org/systemd/systemd/tree/src/core/service.c#n2783.
Fixed in http://cgit.freedesktop.org/systemd/systemd/commit/?id=18abe7bd3e.
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