Bruce Schneier on overreaching, often ineffective security measures:
For example, press reports make the gun-detection machine look good. We see images from the camera that pretty clearly show a gun outlined under someone's clothing. From that, we can imagine how this technology can spot gun-toting criminals as they enter government buildings or terrorize neighborhoods. Given the right inputs, we naturally construct these stories in our heads. The technology seems like a good idea, we conclude.
The reality is that we reach these conclusions much in the same way we decide that, say, drinking Mountain Dew makes you look cool. These are, after all, the products of for-profit companies, pushed by vendors looking to make sales.