At one time the airline where I worked operated a flight from Bahrain to Riyadh and then to Hong Kong. The standard practice was to tanker as much fuel as possible out of Bahrain and plan to land at MLW in Riyadh. One night we received an estimate of the ZFW that seemed quite low, but after being told that it was correct we did the usual thing and bunged on the fuel, less a bit to cater for last minute changes. All well and good, except that when we received the final ZFW it was WELL over the original estimate. Oops! Fortunately, the engineers managed to find an empty tanker fairly quickly, but we nevertheless incurred a delay of several hours to de-fuel the aircraft.