Aerodynamic losses are influenced by the shape or geometry of an object, its cross sectional area and most importantly the velocity of the object or airflow. The combination of these influences result in counter productive forces caused by pressure distributions and/or skin friction.
Examples where this may occur:
Vehicle features: spoilers, air foils, inlet ducts, belly pans, diffusers, grill openings, mirrors, antennae's, etc.....
Powertrain features: Cylinder head ports, valves, intake manifolds, throttle bodies, carburetors, air filters, superchargers, turbos, headers, etc.....