I saw this elegant series of elevator buttons in a building I visited a couple of weeks ago. Most buildings run with B1, B2 or similar for their underground levels, and a G for Ground or L for Lobby.
If you think about it, it makes perfect sense to call the ground floor level zero, and the underground levels should be negative numbers. A moment’s thought about it and you’d work out what was happening, but it’s obviously proven to be such a problem that someone in the building has added the sticker onto the interface to save people the trouble.
When you’re working out how to label everyday things, it’s not necessarily time to be clever at the expense of usability.