Ceiling fans. Two chains: one for the light, the other for the fan. I’m pretty sure at one time or another, we’ve all wondered, “Which chain do I pull?” The fact that we have to think about it demonstrates it’s poor design.
When designing controls, the controls should differentiate from one another in some way. Not only should they look different, but they should reflect their function. As we just learned from Cooper (2007), designers should design to represent the mental model or vision of the user. There may only be two controls, but it doesn’t mean that product designers should just assume that users will remember the function of each. Perhaps one should end with a light and the other with a fan, to reflect their uses.