Forgot to mention another quirk of SMGs. If the loads are too light (usually if handloads are used) the bolt will not blow back far enough to be engaged by the sear and the resulting firing cannot be stopped by releasing the trigger. This will occur regardless of the position of the selector.
As info, much of the 9mm ammo the U.S. sent to England in WWII, was so weak the STEN could not be "shut off" until the magazine was empty. The British marked the stuff "NOT FOR STEN" which some American perversely believe means it is too powerful for SMGs, when the opposite is true.