Brakes Failed problems of the 2007 Mazda MAZDA3

Emergency/parking brake failure I parked my car in the driveway at my mothers house at the top of the driveway, which is relatively flat and only a very slight incline. There is a portion of driveway between the sidewalk and the house level which is steeper; however my car was parked above that area and on the flat portion. The car is a manual transmission and I had set the car to neutral with the emergency parking brake on. About 20 minutes after the car had been parked, it rolled backwards- down the driveway, out into the street and hitting 2-3 trash cans, and ended up jumping the curb on the opposite side of the street. The rear left tire was up on the curb and the Mazda 3 collided with the rear right corner of another parked vehicle. I witnessed the incident from inside the house and there were several other witnesses. When we looked at the Mazda 3 after it had come to a stop on the opposite curb, we saw from the outside that the emergency brake was indeed engaged. Background information on vehicle use: I always fully engage the brakes before parking and setting the emergency brake. When parking on any obviously sloped surface, I always put the car into gear (into 1st when sloped upwards, and into reverse when sloped downwards) and set the emergency brake, as well as turn the wheels toward the curb when parallel-parked. I have kept up with normal scheduled maintenance and have read through the owners manual which does not mention anything about when to inspect the emergency brakes effectiveness. I had the issue inspected by the Mazda dealer 2/7/11 and they said they "could not duplicate my issue", also that everything was in proper working order. All they could do was adjust my parking brake (which does feel tighter now). I asked if there is anything I should do differently when I park. They answered that I have been using the correct parking procedures.