The Oral-B "powered" toothbrushes have
unusual buttons for controlling the on and off functions. You have
to turn it on to start it - and you have to actually start it
without looking at it: while it's in your mouth. What's unusual
about the button:
In the case of the toothbrush in question, they used a "+" for
on(and, naturally, a "-" for off). This seems wrong to me. If you
could continually press it to turn it up to a higher level, then it
would make sense. But there is only 1 setting, on or off.
But it works. So the design communicates its function. It makes
sense. You don't have to think about it. It's "wrong" in a pure
sense, but who cares: it works.
Design of buttons and knobs (or anything that functions) is
about communicating the function. This can be done with texture,
shapes, symbols, and words. Even on a lowly $10 toothbrush.