Many years ago I was on a BA AGP-LHR A321 that went tech just as the plane was ready to pushback (some component failed when the ground power was being detached from the plane) and a replacement part shipped overnight.
Loads of pax either took the LGW flight that was departing in a couple of hours or an early morning one.
When the re-scheduled flight took off the next morning (after repairs), the cabin had max 60 pax which felt quite empty for the A321.
Cabin crew just told us to pick whatever we wanted from the BOB menu as it was “on the house”.