I've never (knock on wood) done this, but my daughter called me one day to report that she had hopped out of the Mustang at the stop sign in front of our house to install the radio antenna. Locked out and blocking traffic. I knew there was no spare key anywhere, and the engine was running, so I had her break out the quarter window on the passenger side. The glass company told me that was probably the most expensive and hardest glass to replace. D'OH!!!