I've noticed this too! I have pulled into my garage, came to a stop with brake fully depressed, pressed P, then held my brake longer than a few seconds (normally I take my foot off the brake about 1 second after P is pressed (since the car is in park)) because I was distracted by the radio, gathering things in the car, etc. and then taken my foot off the brake and the car jumps forward (I'd estimate less than a foot--maybe a few inches) violently.
It freaks me out because I immediately reapply the brake and look down to the shifter to see if I forgot to place the car in park....but lo and behold, every time it lurched forward, the car shift indicator was always in park. And I can't recall this ever happening right after I shifted to park (which might be expected if I let off the brake prior to park fully engaging)...I'm virtually certain it happens after I have delayed releasing the brake pedal at least several seconds after pressing park.