Maybe the take off was attempted with the autopilot engaged on the ground. This has happened once before in a 777, off 12R as I recall. I believe the pitch mode was ALT and the ASR quoted the pilot as reporting "higher than usual stick force" required to rotate the aircraft. It was not a ULR flight so although they used a lot of additional runway they still didn't hit anything. Washington would be a different story. I'm not saying this is what happened but it is reminiscent of the other event and to the best of my knowledge nothing was changed to prevent it happening again.
This one crossed my mind as well. I have personally experienced something like this in a 777. AP was engaged all the time at the gate, and we found out it was engaged when we did the flight controls check after engine start. This disengaged the AP due to the control forces needed, and the wailer went off when it disengaged.
In our case it turned out that maintenance had engaged it to do some work, and had left it engaged, and left the cockpit when we arrived before the flight.
Didn’t AirFrance attempt a takeoff with the AP engaged in Port Harcourt with a 777? Thought they did…
EDIT
Found it. A/P engaged takeoff attempt:
http://www.avherald.com/h?article=428c9bdd/0000&opt=0