Kyrie Irving's serving up hard life lessons to young hoops fans, including the golden rule: With great seats, comes great responsibility ... to duck when loose balls come flying at your head!!!

Kyrie's Dallas Mavericks were playing the Boston Celtics Saturday night, when he uncorked a wild cross-court pass that missed his teammates ... but nailed a kid who was sitting courtside.

The poor young fan wasn't looking when the basketball blew up his world, leaving the kid dazed and confused for a few moments. Mavericks staff quickly tended to the fan, whose nose seemed to be bleeding.

Little man took it like a champ, though, and after a few players -- including Professor Irving -- came over to check on him and give him a few attaboys ... play resumed.

Well, at least the kid got to see his team win -- oh, wait ... the reigning champs crushed the Mavs 122-107.

It's a hard knock life!