The failed launch of my first high-power rocket. After the launch, I found out that the o-rings in motor burned through, and hot gasses shot up through my rocket. This not only set fire to the lower half, but ignited the explosive charge that was supposed to deploy my parachutes, causing the nose cone to pop off just after launch. Fortunately, the upper half, including the nose cone, payload bay, and parachute, survived and floated to the ground about 300 yards away. Had the rocket not exploded, it probably would've floated far away into the nearby forrest, and I wouldn't have recovered anything.