I generally dive with friends; we have recreational gear. Out of habit, I generally check their valve position, and sometimes even check the tightness of the OPV valve cap on the BC. I've prevented a lot of incidents like this and I think this is why many DM's (in recreational settings) are going to check the valve position, before the diver splashes.
If they are good at it (sneaky I mean) the poor unsuspecting diver never even knows it happened. I am sure from the DM's perspective checking prevents a lot of problems, especially when they keep finding valves that are not on all the way.
Also it is IMPOSSIBLE for me to accidentally turn a tank valve off on a buddy, because I do not actually turn the valve handle more than maybe a quarter or half turn. I simply check the position.
I try to shut it off, and if it moves that way, then I turn it back to on to the stop (fully open) and then back a sliver. If they have it off, and I try to turn it off further, the valve is not open since it doesn't move.
If they have it only partially open when I check, then it may stop when I try to close and it certainly will not stop when I turn the other way and attempt to detect fully open.
The whole thing takes only a moment.
Regardless, of what I find, I pretty much return the valve to the original position.
If I THINK there is a problem, then I will say something to the buddy like..."hey I think your tank was off (or only open a tiny bit) ; do you want me to fix it or do you want to check yourself?" Invariably, they say "can you fix it please". And I crank it open and then ask "is your tank full" watching that they check the pressure.
I can remember only one guy who objected, and I kinda laughed at him and said "you are going to trust me to drive your boat while you are down and not run you over when I pick you up, yet you don't trust me with checking a valve?, LOL".