Write down your name, insurance information and number and leave it underneath the windshield wipers.
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The driver of the car that was driving when the accident occured. The owner of the parked car should not be responsible unless the car was parked in an illegal place or position.
Leave your name and phone # on the car so the owner can contact you.
The owner of the parked car is at fault if they open their door into traffic. Their insurance should pay for the damages to the driving car.
Leave a note with your name and number if you can't find the owner.
The owner of the car that was wrongly parked still has the ability to sue. They should get the other driver's information and file a claim on their lawsuit.
leave your information and take lots of pictures. If there is damage, they will contact you. Call the police
Call the police, they can contact they owner for you and help you get everything ironned out.
Leave a note on the windshield with your contact info
unfortunate for the owner of the slider, they are responsible.
From your question I am perceiving that you parked straight and someone came and parked beside you but the rear of their car is blocking the rear of your car. You would be at fault because you should have realized that you wouldn't have been able to back out. In that case you should have contacted the local police (don't call 911) to have them do something with the car if you are not able to find the owner of the vehicle.