Hello. I have a 2001 S500. The car has been running beautifully now for over a year without incident.
However my problems were far more complicated than you are now experiencing. Do a search on "S500 home at last"
This may be helpful. Worst case senario is your electronic gear box is dirty. The sensor can not read the position of your selection. DIY cleaning is not an option. But that is what I was told.

If this is actually your problem, be thankful it occurred while the car was stopped. There have been incidents where the dirty sensor/gearbox slammed the trans. into reverse while people were at freeway speeds.
Check TSB (technical service bullitens) on your make and model to uncover any other issues you have not experienced yet.

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