I once discovered that a paperclip had been taped to one half of the magnetic door seal of a former employer's NOC. It was up at the top of the door, so it wou

I once discovered that a paperclip had been taped to one half of the magnetic door seal of a former employer's NOC. It was up at the top of the door, so it would never normally enter your line-of-sight, so it was very unlikely to be noticed. After thinking about it for a moment, I put my shoulder into the (locked) door and confirmed my suspicion as to why this had been done: the additional space between the magnets weakens the field just enough that you can ram the door and pop it open, but it doesn't weaken it enough that anyone would ever notice otherwise. Pretty clever!