Also chiming in... It happened to me twice this weekend. Creep mode off. P85D delivered last month. SF Bay Area, so warm weather.
1st time: Saturday. On a downward incline. Put the car into (R)everse. Press accelerator, but nothing happens. The car rolls forward. I panic a little and press on brake, press (P)ark. It happened pretty quick, I don't remember all the things I did. After putting the car back into (P)ark, it works normally again. So I assumed it was user error (I must have put it into neutral?).
2nd time: Sunday. On a upward incline this time. Put the car into (D)rive. Press accelerator, but nothing happens. The car rolls backward. I pumped the brakes multiple times. The car stops. I put the car back into (P)ark. Then it works normally again. Can't reproduce the error, so could it be user error again?
After reading this thread, I'm pretty sure it's not user error.
I contacted Tesla.