I have a MK3 so no ACC, just CC so it will keep constant speed and drive into the back of someone if I'm not paying attention. I suppose the Collision Detection would step in at that point as I think it's the only override system on the car. It has lane keeping too, but it only makes a noise rather than taking any corrective action so I tend to keep it turned off
The other week I was driving in town and passed a 30 sign which had been vandalized with spray paint so the car read it as 80 in the information display which isn't even a valid UK speed limit.
I did wonder if the car was self-driving if it would have sped off at that speed through town!