Toggles
A thousand times yes to this post from Lukas Mathis regarding toggles in user interfaces, especially if all toggles shown are in the same state:
Is this toggle on or off? If you don’t already know that “left” means “off” and “right” means “on”, you have absolutely no way to tell.
My favourite is this one that’s in a hideous bit of enterprise software I have to use every day:
It’s been like this for years and I still have no idea which thing is active.