I have dialled up the wrong figure on the fuelling panel before now, e.g. 12500 instead of 13500, by mistake, through mis-remembering the figure. (Error spotted and correct fuel loaded before push-back, I hasten to add).
Equally, one might load too much fuel by mistake, leading to being overweight at destination. When recording the uplift and literage in the tech log, the error would have been spotted, but by then too much fuel would already be on board.
Another reason could have been an aircraft swap. Plane was originally fuelled for a long flight but then some tech issue arose, meaning it could not fly the long sector and was swapped to a shorter sector, but only after the fuel had been loaded. So it would arrive overweight for landing, and they would fly the sector at lower levels to try to burn off the excess fuel.
Defuelling a plane is a very laborious and tedious process, since you are not allowed to unload the fuel back onto the tanker it came from, because that fuel might now be contaminated.