I had just jumped on my bike, and was therefore still in our parking lot at work. I was just getting up to the first turn into the drive that leads from Ackerman into our lot, and I was rolling past a row of cars when the big ugly SUV just to my right (and the one I was JUST passing) suddenly starts backing up. I don't remember getting panicked, more angry. The backup lights came on, and it started rolling just as my front wheel was rolling behind it. I lifted my left hand and smacked down hard on its rear windshield with an open palm.
The SUV stopped quickly, and the blond gal at the wheel looks out the window with a look of either "Who the heck did I just hit?" or "Who the heck do you think you are hitting my car?" Either way, I think I got her attention when I yelled back "What the HELL are you doing?" I think she was on her cell phone - she was when I was unlocking my bike and putting the locks away as she walked past me.
Well... I think I did the right thing with smacking the windshield - no damage to the car and it conveyed the message of "Stop before you hit me." Hopefully that girl with the car that was obviously too big for her to handle learned a lesson, too.