The wild scene happened during a break in the action at Crytpo.com Arena ... when Lakers officials set up a game where a fan named Anthony could've scored a ton of cash if he hit a half-court shot.
Anthony initially seemed super confident he'd get the job done ... telling an in-arena host, "This is what I do."
A short time later, however, everyone in the building quickly learned this not what Anthony does.
Check out footage from the contest ... as Anthony released the ball, he somehow hurt his left leg -- and he was hurting so much, he ultimately crumpled to the floor.