Couple of details here:
Toyota had no software testing procedures, no peer review, etc. The secondary backup CPU code was provided by a third party in compiled form, Toyota never examined it.
Their coding standards were ad hoc and they failed to follow them. Simple static analysis tools found massive numbers of errors.
They used over ten thousand global variables, with numerous confirmed race conditions, nested locks, etc.
Their watchdog merely checked that the system was running and did not respond to task failures or CPU overload conditions so would not bother to reset the ECU, even if most of the tasks crashed. Since this is the basic function of a watchdog, they may as well not have had one.
They claimed to be using ECC memory but did not, so anything from single bit errors to whole page corruption were undetected and uncorrected.
A bunch of logic was jammed in one spaghetti task that was both responsible for calculating the throttle position, running various failsafes, and recording diagnostic error codes. Any failure of this task was undetected by the watchdog and disabled most of the failsafes. Due to no ECC and the stack issue below, a single bit error would turn off the runnable flag for this task and cause it to stop being scheduled for CPU time. No error codes would be recorded.
They did not do any logging (eg of OS task scheduler state, number of ECU resets, etc), not even in the event of a crash or ECU reset.
The code contained various recursive paths and no effort was made to prevent stack overflows. Worse, the RTOS kernel data structures were located immediately after the 4K stack, so stack overflows could smash these structures, including disabling tasks from running.
They were supposed to be using mirroring of variables to detect memory smashing/corruption (write A and XOR A to separate locations, then compare them on read to make sure they match). They were not doing this for some critical variables for some inexplicable reason, including the throttle position so any memory corruption could write a max throttle value and be undetected.
Instead of using the certified, audited version of the RTOS like most auto makers, they used an unverified version.
Thanks to not bothering to review the OS code, they had no idea the OS data structures were not mirrored. A single bit flip can start or stop a task, even a life-safety critical one.
These are just some of the massive glaring failures at every level of specifying, coding, and testing a safety-critical embedded system.
I am now confident in saying at least some of the unintended acceleration events with Toyota vehicles were caused by software failures due to gross incompetence and negligence on the part of Toyota. They stumbled into writing software, piling hack on top of hack, never bothering to implement any testing, peer review, documentation, specifications, or even the slightest hint that they even considered the software something worth noticing.