On a very full flight the other day and as they ran out of overhead space the cabin attendant told the FF that because a second carry on would not fit under the seat that it had to go in the hold. He quickly replied that it had camera equipment with batteries and continued to jam it in the overhead.
those in the know get to use overhead space even after boarding late
there was no way with jammed aisles of boarding passengers to sort out each piece of baggage they are storing once they have boarded