When I goth through, I took my jacket, my shoes, my laptop and my bag and got on the plane. Later, I discovered that I have taken the wrong laptop.
Why didn't anybody stop me?
Why didn't I check the laptop better when I got through? Well, it was the only laptop there, because someone else had taken my laptop.
Why didn't anybody stop him?
In practice, two people where allowed to take someones quite expensive laptop without being stopped. I was well prepared, but that didn't prevent my laptop from being stolen when I was "forced" to give up my laptop in the control.