I teach a course with a piece of software that makes liberal use of this checkbox:
I'd say around a quarter to a third of participants completely fail to use it without help. They all try to drag the white circle to the left, whereas the correct usage is simply to click on it - the white circle moves left and it looks like this:
Their behaviour seems completely reasonable to me: it looks like a peg sticking out of a slot, and you slide it to the left.
Are the designers mistaken by making it clickable, or is there another explanation I'm missing?
As noted, the question in the body here doesn't quite match the title question. I guess I'm asking "Is there a valid visual metaphor that corresponds to this design and behaviour, and if not, what has gone wrong?"