Two employees were trying to put an HVAC unit into place for installation. They were using a manual pallet jack to elevate the unit enough to remove four-wheeled dollies from under the unit's legs. They removed the dollies on one side of the unit, but when one of them began to remove a dolly on the other side, the unit began to tip over. The employee suffered a left knee injury while quickly getting away from the falling HVAC unit.