Useful information. Sometimes OBS just hangs (refuses to shut down), especially if one of the sources is not present. That occurs more frequently than I would like, such as when the RTSP feed is interrupted from a remote camera or a source was not plugged in that was present before causing it to fail to start/shut down.
The error first presented after upgrading to the latest version of OBS around October. The error was also inconsistent (appeared on most, but not all attempts to start). I always selected to "Start OBS normally" and presumed a scene or source was to blame.
I fully removed all of OBS along with all settings and %APPDATA%/obs-studio, rebooted my system, reinstalled OBS, added a single source (my dedicated webcamera), shut OBS down cleanly, restarted it, shut it down cleanly again, restarted it once more. On the next attempt, OBS was slow to shut down and, when restarting OBS, the error appeared again. On the next attempt, no error, the "safe_mode" file is definitely created when OBS is started, apparently it isn't always getting removed or OBS just isn't shutting down completely.
Definitely seems to be a bug with the current version, on the previous version(s) even when OBS crashed or had to be force-killed due to a hang, I didn't have this error on startup. I also don't understand why such a mode even exists but, knowing this file always exists, explains why the behavior isn't consistent, something is causing it to be left behind some, but not all of the time, even on a fresh installation with a single scene and a single source.