It is interesting what gets missed on occasion. Some years ago a friend brought a ceremonial dagger into the UK from Africa in the bottom of her backpack. She'd forgotten it was there and it was only when outbound from Heathrow that security pointed out its presence. They let her put it in a parcel and put it in the post.
Flying out of SFO when my son was small, I'd gone for a walk around the duty-free and when I got back to where he and my wife were sitting, he was drinking from one of his favourite soy milk cartons. Which was greater than 100ml by volume and it was part of a six pack. Security didn't spot it that day.
I had a hard disk in my backpack once, security were most interested in it (opaque to x-rays) and it got carefully checked for explosives. Second time, I put it in the tray with the laptop and it went through with no comment.