Adrien Broner -- HUMILIATING MOTOR-SCOOTER FAIL ... On Video!

Adrien Broner got DROPPED in the middle of a Miami street the other day ... after embarrassingly crashing his motor-scooter into a parked car ... and TMZ Sports has the footage.

You gotta see this ... the boxing stud was just beginning a South Beach scooter street session with friends when things went horribly wrong.

Even when he got back on the ride, the 24-year-old had problems starting it up again ... just a total disaster.

The good news? Broner and the car were unharmed.