The car was parked behind another car in the driveway (which is level, not on a hill). The car in front attempted to back out of the driveway and lightly hit the Impala (no damage to either vehicle!). As a result of that slight tap, the Impala rolled out of the driveway and into the street. The car (an automatic) was in park and we feel this should not have happened. We went to the street, climbed into the Impala, and it started up as usual and allowed us to put it in drive and get back into the driveway. We are concerned since the car should not have moved at all since it was turned off and in park.