The parking brake is not optional equipment. It locks up the wheels so the car doesn't roll. It also prevents stripping the parking pawl in the transmission when in "park." The car is not meant to be "held" by the transmission; that is what the parking brake is for.
2. Not Coming To a Complete Stop Before Shifting Into Reverse
You strip the gears and bands in the transmission when you don't come to a complete stop before shifting gears. Eventually, you won't be able to shift gears because they will no longer have teeth to mesh, nor any bands to apply tension to hold all that force in place for propulsion.
3. Forgetting To Change the Oil
The engine has metal pistons with round metal rings that slide up and down inside the metal cylinder. If oil is not maintained, the metal-on-metal rubbing can eventually lead to some major issues.
4. Pressure Washing The Engine
Power washers supply extreme pressure to everything you spray with it, including those closed modules where expensive electronics are housed. A clean engine is nice but at what cost?