Justin Bieber Knew He'd Get Searched and Still Smoked Mary J Up the Jet -TMZ

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Justin Bieber is not only out of control ... he's hard-core stupid.  He knew last week -- before he took off for New Jersey -- his plane would get searched by U.S. Customs yet he and his friends still hotboxed on the private jet.

TMZ has confirmed .... the pilot radioed in the air Justin and his gang were smoking weed on the plane and the flight attendant couldn't get them to stop.  The pilot complained the smoke in the cabin was thick.  NBC News says the pilot actually had to don a gas mask because it was so bad.