This is the kind of thing you would rather see in the pre-demo tests:

In the programme, Mercedes drives 3 S-classes behind each other. The first car hits the brakes, and the two following cars brake down automatically, to avoid a crash. In theory. In practice, the first car braked, and the two others continued right into the back of each other, resulting in a 3-car pile-up at the test site.

According to a German news site, the blushing Mercedes security engineers soon discovered the problem: The test had been done in a hall which was made of steel. This confuses the radar, and the system doesn't work properly, causing more than £150,000 worth of cars to crumple into each other.

The system "works perfectly in all other circumstances", according to Mercedes. For now, though, it may be worth keeping your foot near the brake pedal.

Reminds me of the "Windows Live" demo. If you plan to give a demo with cameras rolling, it's always a good idea to try a dry run in the same place first.