Comment by RealityVoid

No, this is not what causes this. Most ECU's have a BSP package and some drivers bought from vendors, then the tier1's build the whole thing to OEM specifications. OEM's then integrate the whole thing. Stuff starts to break when you put them together. Maybe a diag is slightly different or an ECU has slightly different timing, or one of the gateways doesn't like what a bootloader is doing, or you have some weird race conditions that fail at 1 out of 1000 cases.