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 the White House 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.