I'll NEVER forget footage that I saw of the BTCC (British Touring Car Championship), where I saw two identical Volvos (same race team), barreling down the straight one after the other.
Halfway down the straight, the rear wing (it was one of those cool touring car T-shaped wings) of the second Volvo rip off and tumble along the tarmac. I'm not sure that the driver noticed, because he kept right into his throttle
Eventually, both cars enter Turn 1 and the first car just brakes and the whole car squats down and gets on with his braking. The second car following a little behind also hits his braking marker, but the car doesn't seem to slow down at all!!! The rear of the car just waddles around helplessly as the lack of downforce and resultant lack of grip meant that he couldn't brake ANYWHERE NEAR as hard as his teammate, let alone attempt making the first turn. He ended up squirming all over the place, and spearing straight off the end of the track at some RIDICULOUS speed, as his teammate casually took turn 1 without a care in the world.
A perfect example of two cars entering a corner with (nearly) exactly the same setup, at the same speed. One with a rear wing, and one without.
I wish I could find the footage to show you guys