Here's an "interview question" that while ostensibly about hardware really does inform a software design principal as well.

Computers used to (still do I guess, somewhere) use magnetic tape reels to store data. There was a plastic accessory you could attach to a tape reel called a "write-enable ring". If the tape had such a ring, the tape drive allowed writing to the tape... if not, it only allowed read access.

Why was the choice to design the system in this way? Why not have a "write protect ring" instead, with the opposite effect?