[wh1t3bread]

The answer to this depends on the state.

If you hit an unattended vehicle in Virginia is is YOUR responsibilty to do the following:

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The driver of any vehicle involved in an accident in which no person is killed or injured, but in which an unattended vehicle or other unattended property is damaged, shall make a reasonable effort to find the owner or custodian of such property and shall report to the owner or custodian the information which the driver is required to report pursuant to § 46.2-894 if such owner or custodian is found. If the owner or custodian of such damaged vehicle or property cannot be found, the driver shall leave a note in a conspicuous place at the scene of the accident and shall report the accident in writing within twenty-four hours to the State Police or the local law-enforcement agency. Such note and written report shall contain the information which the driver is required to report pursuant to § 46.2-894. The written report shall, in addition, state the date, time, and place of the accident and the driver's estimate of the property damage.


[/ QUOTE ]

If that is not done and you get caught then the penalty is as follows:

[ QUOTE ]
Any person convicted of violating the provisions of §§ 46.2-895 through 46.2-897 shall, if such accident results in injury to or the death of any person, be guilty of a Class 6 felony. If such accident results only in damage to property, the person so convicted shall be guilty of a Class 1 misdemeanor; however, if the vehicle or other property struck is unattended and such damage is less than $250, such person shall be guilty of a Class 4 misdemeanor. A motor vehicle operator convicted of a Class 4 misdemeanor under this section shall be assigned three demerit points by the Commissioner of the Department of Motor Vehicles.


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So basically as long as no one was hurt and the damage is minimal you get a couple points and fine.