|video| B-Real From Cypress Hill Learns "Don't Take Joints from Strangers!"

Rapper B-Real from Cypress Hill learned the hard way -- DON'T EVER SMOKE ANYTHING HANDED TO YOU BY A RANDOM FAN!!!

Mr. Real was leaving Fogo de Chao in L.A. this weekend -- and explained he once made the mistake of puffing a mystery doobie at a concert in Amsterdam ... "the worst thing ever."

Now, B says he's got a guy who handles reefer of unknown origin ... and you'll never believe what they do with the stuff.