It also is "unpredictable" what happens when you overrun a stack frame. There is zero guarantee that the infamous Sufficiently Sophisticated Attacker couldn't

It also is "unpredictable" what happens when you overrun a stack frame. There is zero guarantee that the infamous Sufficiently Sophisticated Attacker couldn't predict it. Hardware is largely deterministic, even when it doesn't behave in the documented way. I wouldn't interpret this as literally unpredictable, it's just a generic slogan they always use in their errata. And they aren't going to say anything more for obvious reasons. Patch this damn microcode.