The handles for operating the sink and the shower valves in Statler Hotel bathrooms use nearly identical design language:
But they do different things! In the sink, the two handles control the amount of hot and cold water, respectively. In the shower, however, the left handle controls the volume of water while the right one is used to control the temperature:
This design illustrates why consistency is such a hard design principle to get right: on the one hand the shower controls are consistent with the designs of other showers in other bathrooms; on the other hand, the design of the shower controls is inconsistent with the design of the sink controls in the same bathroom. So is a completely consistent design out of reach here?
The problem in this case is caused by both the sink and the shower controls looking the same (apparently in an attempt to make the visual design throughout the bathroom consistent). The use of exactly the same visual design (material and shape) creates an expectation of similar functionality. A simple solution would have been to give the shower controls substantially different shape (perhaps by making them larger, or by making the resting positions of the handles vertical rather than horizontal) while using the same materials and colors. This would have preserved the consistency of visual design, while signaling that the two sets of controls are distinct in function.
This example was contributed by Prof. Harry Lewis.