I've recently finished fifteen years of motorway commuting. I've lost count of the occasions when I've seen cars either stopped on or, even more frightening, reversing up slip roads. Presumably, they were on a roundabout (both the junctions I used regularly were both motorway access and other roads), took the wrong exit, and were so paniced by the idea of driving on a motorway that they thought stopping, or reversing, was their safest course of action. Or they can't navigate and any deviation from their assigned route inspires total panic (in both cases, the next junction is about two miles, so the next result of carrying on to the next junction, going round the roundabout and coming back is five minutes' delay, ten if the traffic's a bit heavy).
This week however took the "stupid nervous driver who shouldn't be on the roads" biscuit. End of the Aston Expressway at Spaghetti Junction, you have a variety of options, including the M6(N) and the M6(S). Someone got into the lane for the M6(S) and, after passing the slip to the M6(N), decided that was where they wanted to go. So they stopped. Indicated left. And turned dead slow across three lanes onto the northbound M6. For fuck's sake: again, it's about ten minutes to go to the next junction and spin round, and instead you attempt to kill everyone around you. It was a total miracle, and a testament to modern brakes, that no-one hit them (I was watching from about 400 yards back, so had plenty of time to stop).
I passed them, or a very similar car, about ten minutes later, stopped on the hard shoulder, speaking to the nice policeman, so presumably someone watching the CCTV had been alert.
If you can't recover from taking the wrong turn without endangering both yourself and everyone else, you really shouldn't be on the roads.