I think horizontal shoulder holsters just aren't generally worn much by people that carry a lot. I've never met a cop that does, on or off duty, including the tactical team guys. Sonny Crocket carried one I guess...
So with that I generally assume that if I came across someone at the range (or anywhere else) carrying in that manner, he's less than well trained. True the gun in the holster won't obviously "go off by itself". But at some point that guy will take it out of the holster. When he does it's a guaranteed 180 degree muzzle sweep at least. During that time he may or may not have his finger on the trigger. That doesn't matter much to me. Sweep me with a loaded (or even unloaded) pistol, even with your finger off the trigger, and I'm nervous.
Mental breakdown? Um, no.
Sgt Lumpy