First thing, if the car is in D, and brakes are not pressed, the car will still move slowly and you don't know what's happening(Happens with people who are driving an auto car for the first time). Plus I don't know when ever I press the accelerator of an auto car little bit, it goes wild compared to a manual one. Plus the clutch thing, you might know that there's no clutch in an auto car, most people who are driving an automatic car for the first time without any experience accidentally press the brake paddle in the middle of the road, I did that once but thank God no car was behind! The gear part will bother you again and again, you'll try to change the gear as if you're driving a manual car and all that stuff. If you are cautious nothing will happen.