I got stopped by the buzzer in DCA earlier this year. Boarding was in process, and I confidently strode through the group 1 lane and handed my BP over in front of the lowly group 3's waiting to board, and got the dreaded buzz. I can't even blame a gate change. My flight had always gone out from this gate in the past, and I just wasn't paying attention. It's a lesson I have to relearn every couple of years.

COPA GA's often just take the BP and not scan, and I always double check with the flight attendants when they do that.