When you undergo gastric bypass surgery, NEVER TRUST A FART -- and Al  Roker learned that lesson the hard way ... at the White  House.

Roker -- who got his stomach stapled in March 2002 -- shared his  most embarrassing moment on "Dateline" last night, saying he was covering an  event at 1600 Pennsylvania Ave a month after his operation ... when he felt the  urge to let one rip.

Roker said, "I probably went off and ate something  I wasn't supposed to. And as I'm walking to the press room, [I'm thinking] well,  I gotta pass a little gas here. I'm walking by myself. Who's gonna know? Only a  little something extra came out. I pooped my pants."

Roker said he  beelined for the restroom to dump his underwear in the trash -- and proceeded to  go commando the rest of the day.

The lesson: watch what you eat. The  other lesson: carry spare underpants

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