I have pin 4 high, but I have a check to tell me if the values change in the tx register while it's being copied. I want to know that it found something instead of silently fixing it.
And it fails. Not as often, but it does fail.
I've been noticing though that I interact with the internet. My device is subject to the vagaries of the world and I could well be getting partial packets and junk from time to time.