The California Highway Patrol (at least in one case) says floor mats. Toyota says gas pedals...a university professor says electronics (but Toyota says it couldn't work the way he says in real life)...the feds say maybe radiation from outer space, Jim "Runaway Prius" Sikes said the brakes don't work (but Toyota and the feds' investigation said otherwise) and at least one driver says demonic possession.
So what's really causing unintended acceleration? Find the cause, prove it and get rich. As in a million dollars. Edmunds.com is running the contest. Details here.
Gentle reminder: Most fully-investigated cases have shown drivers were standing on the gas when they thought they were on the brake.