The main problem with going through the archway with a bag or bangles etc etc by mistake is that security has to then body search because the gate has alarmed despite knowing which offending article has triggered the alarm. (they are viewed on cctv and can't just let you go because they thought the alarm was your bag etc) . Obviously they try to minimise the amount of searches by getting people to remove items that set the alarm off, this means they can concentrate on real problems rather than someone just forgetting they have something in their pocket etc. thus the reason for taking nearly everything off to go through the x-ray! if they didn't do this they would end up body searching nearly everyone which really would slow things up.
I must admit that when i go through security my mind has wandered into a world of its own, there is a lot going on and i think most people find themselves concentrating on one thing and forgetting another,
which brings me to an incident that happened to one of my wifes colleagues , he was on the belt loading items into the x-ray machine and looked down the queue and noticed an attractive lady with a seriously ample chest, and everyone in the queue was aware of her aswell. He asked for her hat and sunglasses, bag and bangles and belt, he was trying to look elsewhere but it was difficult not to notice what was exactly in front of him, he could also feel everyones eyes on him aswell , she walked to the gate and a female officer stopped her , and called him and said , you didn't ask for her shoes! feeling flustered he replied oh sorry i couldn't see them because of her big chest, everyone in the queue was laughing , and the lady didn't mind either after making him feel a bit uncomfortable for a while. so there , its easy for anyone to lose their concentration even if they think they know what they're doing.