Things like that just happen. I once had a DH8C with only small payload planned for a flight to a field where fuel was extraordinarily expensive. So we planned to tanker fuel for both legs, leaving several 100kg margin to the limiting MLW. Along came an unexpected travelling party of 20 people or so. Landing weight was still looking good on paper, but we had to burn off no less than the planned fuel to get below this limit. Knowing that our flight planning system at that time erred generously on the conservative side, we were unable to accept any of the normally substantial directs offered on the route and even had to drop the gear and flaps 30 miles out in order to get rid of the unwanted fuel (the DH8 FMS does the fuel prediction based on current fuel flow and speed; while this has its problems as well, in this situation it was helpful). And for the way back, we had to take some of the expensive fuel.
Was this economic? Fuel wise, definitely not. But most probably it was still a better option than leaving paying passengers behind.