For me one that sticks in my mind is the occasion where a 'Steam Dreams' special tour in the 2000s (2002 I think) added stops between Bournemouth and Weymouth to cover for a failed SWT service train. Thus we had ordinary passengers using the steam (BR Standard 'Five' 73096) working, with green slam-door coaches, for their normal journeys!
Further back still, in January 1982, I recall a snowy spell when I was waiting at Sutton to get home to Epsom. Many cancellations saw a Victoria to Bognor (4CIG, 4BIG, 4CIG) working given an extra stop at Epsom. I happened to board next to the buffet car and bought a coffee while doing well over 80mph (before the days of over-speed controls!) to Epsom, just to be able to say I had used a buffet car on a service train between Sutton and Epsom!