This is a story as told to me in a bar, so take it for what it is worth!
A particular BOAC VC10 always performed much worse than her colleagues, very sluggish on the T/O roll, used more runway, burnt more fuel, landed more heavily etc. etc. When in the hangar for a check it was discovered that she was carrying several tons of water that had some how accumulated between the floor and the skin but thanks to Vickers engineering had not leaked out or screwed up the electrics or whatever else ran through that part of the aircraft.
Possibly urban myth but I got it from the BOAC Station Engineer in AUH at the time!