The Diaz brothers were playing with fire Sunday night -- hanging backstage at a Snoop concert ... with Nate holding (what appears to be) one of the BIGGEST JOINTS EVER!
No word if Nate actually smoked the joint -- or if it actually contained marijuana -- but it's a troubling image considering Nick was just reinstated following an 18 month ban after testing positive for weed.
Hey, maybe it's just a prop ... right???