Refuelling hiccups
It may not be relevant to this case, but as a former pilot I can remember sometimes getting slightly confused with the complexities of valve operation during some refuelling operations on various airliners. (Do I hear sniggers from former F/Es?) Although 2-pilot aircraft have automatic fuel distribution during refuelling, there are times when adjustments have to be made. These may involve using defuelling valves and tank pumps to shunt the fuel around. And then there have been occasions when crews have tried to squeeze a bit of extra fuel in by one means or another, when range is being pushed to the limit...
As I think sevenstrokeroll pointed out some days ago on another thread: the trouble is that on some aircraft, once fuel starts pouring overboard from the vent-surge tank, a syphon is set up that tends to persist for an embarrassing period. (This can even happen if you have correctly loaded full outer tanks with cold fuel on a sunny day.)