Waka Flocka Flame -- Slam Dunk Fail ... Caught On Video

Waka Flocka Flame is usually a BEAST on the basketball court ... usually ... but the guy ate it hard during a dunk gone bad this week ... and TMZ Sports has the video.

It all went down Tuesday night at the OC Observatory -- where the rapper decided to try out some dunks after performing a set.

He missed.

Twice.

And to make matters worse, he slipped and fell at the end of the second dunk.

Don't worry Waka, even the pros can't get it right all the time ... just ask Shannon Brown.