About 5 years ago was due to fly YYZ-LHR with Air Canada, on a 763, on the "daytime" crossing that leaves YYZ early morning and arrives LHR late evening.
After boarding, noticed that outside some chaps in white overalls were trying to capture liquid emanating from under outer section of stbd wing....looked like a fuel leak.
After a short while, Capt briefs us in very detailed fashion that there is indeed a "small fuel leak, which I would live with if on a short domestic flight, but not as we are going across the ocean".
Within 30mins an identical 763 appears on the adjacent stand, and within another hour we are all transferred (straight seat swap as same config aircraft) and on our way.
Very well handled, and makes all the difference as a pax when kept fully informed and aware that safety was put in front of keeping to sched
(I heard absolutely no one onboard complain about the delay).
Guess the fact that we were at a main base for AC helped of course, but good job none the less.
When we arrived at LHR (having made up a bit of time on the crossing), that very nice AC Concierge who used to feature on the "Airport" Programme was there to greet us and apologise for the delay as we left the aircraft....impressive service.