From a stop, the Hummer began to roll forward. I depressed the brake pedal four times, then quickly put the gear into park. The Hummer continued to roll with no response to stopping. I bumped into the rear of a Ford Focus. Slight damage to rear bumper of Ford. I had the truck looked at by a mechanical shop and could find no reason. I then took it to the brake shop and they also could find no reason. This has not repeated itself.