At times in the UK it is difficult enough to get the standard things accomplished during a turnaround let alone get something non-standard performed, like de-fuelling. As soon as you make a such a request, the average dispatcher will look confused, and then tell you it's not possible and then play the "elf'n'safety" and/or "airport regulations" card/s. For the greater good, it is better to disappoint and really annoy people who have paid and fully expect to fly by off-loading them whilst delaying those who will be flying.
How do you get over-fuelled? I've had the following:
1. Not reading my estimated ZFW (correctly) and then ordering too much in an attempt to save money.
2. Number spoonerisms (if such things exist), like ordering 5,700 kgs and ending up with 7,500 kgs.
3. Finger trouble on the refuelling panel.
4. Tanker faults. I've seen one pump until fuel was vented overboard through the fuel vents.
5. Poor or miss-communication.
6. Aircraft fuel system problems (both hardware and software) whereby the aircraft doesn't shut the fuelling valves when it should. This appears to happen rather a lot on electric aircraft.
7. ...and so on.
Solution: For flight deck and engineers - When fuel really matters watch the refuelling process and for tanker drivers, calculate the expected uplift before refuelling and stop the pump when this figure starts to be exceeded.