Shoot...I have tried to open the doors of trucks that were not even the same color as mine. I even got in one a few years back and wondered why I left my door unlocked until after I had gotten in and realized I was sitting in some other guys truck. It was a little embarrassing....glad nobody shot me.
Here is a thought pattern for you. I hop in the car thinking it is mine. I see you sitting in my passenger seat pulling a gun. I immediately grab your arm to disrupt your draw, grab my gun(that I illegally carried into the post office, because nobody could see through my clothing) and put a double tap into your face. OUCH!!!! Talk about embarrassing moments!!!

I would just feel awful once I realized it was your car instead of mine.
