I was in line behind a girl flying from Schiphol to the US a couple of years ago, and they told her she needed to throw away the unopened bottle of water she'd purchased from a shop in the airport. She was arguing that she'd bought it right there and it hadn't been opened, and the guy said, "they're supposed to seal it in a bag. Maybe you can go get a bag?"

At that point, the rule isn't "you aren't allowed to bring liquids onto a plane because it might be a bomb". The rule is "you have to bring your bomb on the plane in a special bag".