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 ... the suds went all over the man's suit -- and some shrapnel even got on the guy next to him!!

To their credit ... both guys took the spillage in stride, smiling and laughing about it.

Hopefully someone replaced the drink ... 'cause that stuff ain't cheap in the NBA.

As for the game, the Celts ended up losing 129 to 120.