Tough scene for a super rich guy sitting courtside in Boston on Tuesday ... Kemba Walker drilled him in the chest with an errant pass -- SPILLING HIS BEER EVERYWHERE!!
It all started when the Celtics superstar was trying to make a sweet play in the lane against the Brooklyn Nets ... when he fired a bullet to his teammate for an easy layup.
The problem? Romeo Langford couldn't handle the hot potato ... and the ball went crashing right into some dude's beer!
Check out the aftermath of the cold one's explosion .