It can happen with automatic trans that if parked with a stress on the driven wheels (such as on a slope) that the trans (and sometimes the steering, and ignition cylinder) will lock, especially if the wheels are fully right or left. Solution is to block the front wheels and lift the rear so that the torque in the driveshaft is relieved. Had to do it for a frend's car once, worked a charm.
Suggest the parking sequence on slopes should be something like: stop the car, engage the parking brake, foot off the service brake (car may roll a few inches) and only then shift to park.