Britney Spears -- Totally TOXIC at TSA Checkpoint (VIDEO)

Britney Spears was THAT person at the airport -- the one who has you angrily mumbling to yourself about how you're going to miss your flight ... but at least she was entertaining while she held up the line.

Brit and her boys were flying out of LAX on Thursday ... and mama Spears' attempt to get through TSA was a comedy of errors -- once, twice, THREE times she tried and failed to get through the metal detector.

They eventually bumped her over to the full body scanner -- and even that did NOT go smoothly. You gotta see Brit's reaction when she wonders if something under her cowgirl hat is setting off the machine.