"Michael Barr of the Barr Group testified that NASA had not been able to complete its examination of Toyota's ETCS and that Toyota did not follow best practices for real time life critical software, and that a single bit flip which can be caused by cosmic rays could cause unintended acceleration. As well, the run-time stack of the real-time operating system was not large enough and that it was possible for the stack to grow large enough to overwrite data that could cause unintended acceleration."
So... maybe the computer could have physically failed, or a stack overflow, or some other error? Regardless, I still feel like drive-by-wire is scary. The Revisionist History podcast also covers the unintended acceleration recall from more of a psychological perspective[2]
[1] https://en.wikipedia.org/wiki/2009%E2%80%9311_Toyota_vehicle...