I've done that. The 560 Halifax - Commons used to have some runs done by First Manchester drivers in between runs on the 562 Oldham - Halifax. I was the last passenger about to get off in Ripponden and the driver asked if I knew where he was supposed to go after Rishworth. I told him I could show him the way if he liked and he gladly agreed. Good job I went as he wasn't really sure we were going the right way for the last mile or so of the route - and didn't know the bus had to do a three point turn at the terminus. No other passengers boarded on the way up though a few did as he returned and dropped me opposite where I'd originally planned to get off. Not sure what would have happened if an inspector had boarded as unusually I'd bought a single ticket rather than having a pass
A similar situation when I was working for London Transport in Bus Schedules. I noticed that, in a schedule change, a new garage had come onto a route on Saturdays only. As this new garage, Clapton, was in N.E. London and the route being operated, the 47, was primarily in S.E. London right out to Farnborough, almost to the Kent boundary, and their crews had never operated in this part of the world before, I wondered how good the route training would have been for a one-day-a-week allocation. The main part of the allocation was, as ever, from Catford Garage and,in fact, on perusing the schedule, there was only one Clapton journey out to Farnborough, being the last one of the day to that destination. I was living in Bromley, which was their limit on the other journeys, and caught that last bus in Lewisham and got chatting to the young conductor, asking him how they were finding it in these foreign parts. He said OK so far, but the driver was a bit apprehensive about proceeding beyond Bromley Garage to Farnborough as that section hadn't been covered in route learning. I then said that, if they liked, when they got to Bromley South Station, near my home, I'd nip round to the cab and have a word with the driver. This I did, and he said he'd been told by a Catford driver to 'just keep on going along the A21 and you'll come to it' which I realised would have landed him in Sevenoaks if he wasn't careful, there being a right turn to Farnborough which, in those days, was poorly signposted and difficult to see in the dark. I then suggested staying on to make sure he found the turning and the conductor would give three bells about 100 yards before the turning.All was accomplished, turnround was speedy and back in Bromley in less than ten minutes, my good deed for the day done.