I did hear that a steam trip to Lincoln a few Christmases ended up being a diesel trip, I guess the steam loco failed at the last minute, then the diesel ran of of fuel and a farmer in Lincolnshire assisted with a few cans of fuel.
I was on a steam trip to Weymouth some years ago usingthe STD 5 loco (which I think is now at Mid Hants rly), a large nut broke, but luckily enough there was a factory by station at either Overton or Whitchurch which was open on the day, a Saturday, and had a spare nut, which saved the day until a proper replacement was bought to the next waterstop at Salisbury from the MHR at Alton, we were only about 1 hour late in the end and because we delayed SWT between Basingstoke at Salisbury, we stopped at the stns on this line and gave the local passengers a lift to Salsibury. Hopefully going on a steam train eased their slight delay.