Has anyone ever heard of an idling car going from park into drive? It seems that if anything it would go from park into reverse. Is it possible for this to happen?
The person just stated it wrong because details are hard to remember. Many shifters have dropped into reverse. Most will not drop from neutral into drive. It’s really great when the car is being tested by the smog people who can’t follow the directions you give them to not let it idle in park!
Regardless of which of these situations you may be talking about, I would first look at some one NOT leaving the car fully in park before I’d consider a transmission problem. Mechanics aren’t infallible, but are even less trustworthy when used improperly. Also, don’t even leave an unattended car running w/o engaging the parking brake.