Back in college when dollars were few and far between, I tried saving a few bucks by replacing the brake pads on my Honda Accord myself. Parts disassembled easily enough but during reassembly I popped out the brake piston, introducing air into the hydraulic line. Thinking no harm no foul I put the piston back into its housing.
When everything was put together I went for a little ride. I lived atop a large hill with a hospital directly below. As the car descended the hill, gaining momentum, I stomped on the brake pedal that to my surprise traveled effortlessly to the floor mats without slowing the vehicle. Frightened by this trial of fire I began careening down gaining speed and fear. I grabbed the parking brake, which is mechanical and eventually slowed the car to a stop.
I learned a valuable lesson about the importance of brakes and the comfort of living near a hospital.