Justin Bieber -- What's Bong With This Picture?

Justin Bieber may -- or may not -- have pulled a major stoner faux pas in Calabasas yesterday ... 'cause someone inside the singer's van dumped BONG WATER on the side of the road.

MAJOR STONER NO NO!!!!!

It's unclear WHOSE HAND is attached to the bong ... but we do know it's Bieber's ride. He's been toolin' around L.A. in the pimped out Mercedes party van all week long.

Here's what we know ... the bong dumper is definitely NOT Lil Za or Lil Twist ... 'cause, well ... the hand is white. Duh.

We're also told the van was en route to Jim Henson Studios ... and later that day, JB snapped a photo of himself inside a record studio.

Still, since there are no shots of the bong dumper's face ... it's impossible to tell if the perp is Bieber ... or one of Bieber's white friends.

Actually, does Bieber even have white friends anymore? Hmmmmm ...