Depressible buttons on drink carton lids
If you're anything like me (and God help you if you are), you'll have wondered why those little buttons on the plastic lids of soft drink cartons, such as you get from McDonalds, are never actually pressed by the fast food operative. I don't remember ever seeing them used.
However, while enjoying a nutritional* dinner at a Burger King "restaurant", I noticed that the "diet" button on one of my friend's drinks had, in fact, been pressed. After recovering from the initial feeling of jealousy that my button had never been pressed, so to speak, I realised it might have something to do with never ordering diet drinks. That said, some fast food chains have a more advanced drink lid configuration which allows for the flavour to be selected as well, and I've also never seen that used.
But apparently I've been living in the dark, and it's quite common in some countries, such as the United States, to have these buttons used on a regular basis. I can only dream.
If you think depressible buttons on plastic lids are boring, then check this out.
*Burger King helpfully provide a colour-coded table listing all the Good Things contained in each burger, so it must be nutritional.