It happens in the best regulated environments. Departed Manchester one day, bound for a Greek island. Over southern England we were given a routeing totally different from the one on the flight plan. It seems Ops had sent one to us and a different one to ATC. A re-programme of the FMC, and a fuel check, and we were good to go, following the plan ATC had. No big deal, but then we were blessed with a benign route, with lots of alternates if it turned out our calculations were wrong.