I've been called out to departing aircraft a fair few times with 'faults' only to discover a bus-tie switch or fire loop detector switch in the wrong position, enough so that when I'm called to a snag I do a switch check first. These are invariably switches that are always left in their default positions. I suppose if you're checking the same switches every day you become blind to ones that 'never move'. I've been caught out myself on ground runs.
If I'm working a u/s aircraft my thoughts are directed at fixing the fault, not leaving the flight deck switching in a ready to fly state. If I'm working a snag when the crew 'own' the plane, I'll always ask them to move a switch rather than do it myself.
Pre-flight checks just aren't part of an engineer's remit. Harsh as it may be, I'm afraid the buck stops with the flight crew on this one.
Perhaps the problem IS that switches are always left in a flight ready position?!