The Washington Post has a long and interesting article series on US Drone operations in Africa.
The article has this interesting snippet, taken from declassified US Drone incident reports:
Air Force mechanics have reported mysterious incidents in which the airborne robots went haywire. In March 2011, a Predator parked at the camp started its engine without any human direction, even though the ignition had been turned off and the fuel lines closed. Technicians concluded that a software bug had infected the "brains" of the drone, but never pinpointed the problem. "After that whole starting-itself incident, we were fairly wary of the aircraft and watched it pretty closely," an unnamed Air Force squadron commander testified to an investigative board, according to a transcript. "Right now, I still think the software is not good."