To access the RAM slots in a unibody MacBook you must first remove the back plate—a thin piece of aluminum attached by eight tiny screws. Half of the screws are hidden beneath the battery cover, and each one is surrounded by a recessed magnetic zone. The purpose of these zones is ostensibly to help keep the battery cover seamlessly attached to the case.

MacBook magnetic zones

But removing the screws made it clear that the magnetic zones serve a second function. When my screwdriver slipped, the screw didn’t fall into the depths of the case. Instead, it flew right over to the magnet, and I was spared the pain of extracting a three-millimeter needle from an expensive electronic haystack.

It’s a perfect example of real-life defensive design: anticipate where your customers might slip up and try your best to keep them from doing it. Have you encountered any good defensive design lately?