I reckon NASCAR roof flaps would have to be right up there. In the mid-1990s there were some terrible accidents resulting from cars spinning backwards at high velocities on superspeedways and then becoming airborne. NASCAR engineers worked out that when spinning stock cars were facing rearward at about 30 degrees to the airflow, the bodies created the profile of an aircraft wing which generated huge lift that literally sucked cars into the air. The solution was a pair of hinged roof laps (one at 90 degrees, one at 45 degrees) which sat flat when cars were pointing in the right direction (forward). However, when a spinning car reached that critical angle when going backwards, the powerful aerodynamic lift flowing across the roofline would literally 'suck' the roof flaps open, which instantly disrupted the airflow and stopped the cars taking off. Such a simple but brutally effective cure to what was becoming a serious and potentially lethal problem. Roof flaps have since been mandatory in NASCAR racing.