I found it quite difficult to believe too, but a number of posts, from people who appear to know what they are talking about, indicate that this was exactly what happened.
onb - Whatever turns out to be the basis of this incident, it is
almost impossible to set off with an extra 50T of fuel that you don't know about. Before departure you need to:
1) Agree a fuel figure for the flight
2) Notify dispatch, who will use this figure on the fuel sheet.
3) Verify the amount of fuel loaded with the fueler.
4) Verify the loadsheet fuel weights.
5) Check the EICAS fuel loaded figure.
6) Inspect the EICAS fuel page for correct fuel distribution
7) Select the appropriate fuel pumps for the appropriate fuel tanks.
8) Calculate aircraft take off speeds for the known weight.
9) Verify these speeds against those generated by the aircraft FMC.
10) Complete the 'Before Start Checklist' which would sweep up any omissions.
The chances of getting through all 10 steps without spotting an extra 50T you weren't expected are all but zero and would require gross incompetence from a whole chain of people.