Failure in Household Appliances

The image on the left is a depleted air-freshener I found in my friend's bathroom. It's the kind that you plug in and set to release a puff of fragrance at a regular interval. This model had five frequencies but I found the thing to be really confusing. Had it been full, I might have been able to test it but it was empty and so I was just left with elusive knowledge in the world, as Donald Norman would say.

The image may be a bit difficult to see on some screens but the "interface" of the thing has a right-turning mechanism set with the numbers one through five that I'm assuming is used to set the frequency of release. But above is a 'minus' and 'plus' sign with a gradient symbol that increases in slope as it approaches the plus side. The desired frequency is then placed at the notch above and below the turning mechanism. The confusing part though is that the numbers in the turning mechanism start at five and go down. So you immediately see that the five is above the minus and the four is above the plus. Well that doesn't make any sense. Does that mean that five is the lowest setting? Or does five refer to the time (possibly in minutes?) that the frequency is set? If that's true then this device might make more sense because at one puff every minute is more than one puff every five minutes. Still, it leaves the user with something to think about before using it.

If we removed the turning mechanism (which I'm assuming is disk-shaped) and flattened it out so the numbers didn't wrap around, the numbers descend from five to one but the minus to plus signs don't match up. It just doesn't make sense.

A rendering of the unwrapped turning mechanism.
This device implies that five is less than one.

But maybe this design flaw isn't as big a deal as I'm making it out to be. It's not that it's just an air-freshener. It's not that it only costs maybe $4. It's that (at least when it's full) you have a degree of flexibility in failure. If I misuse the air-freshener, nothing bad will happen, even if I fail to operate it fifty times. Of course it wouldn't take me that long, but there are no consequences to misusing it or it not figuring it out. And I'd probably figure it out within five tries, as it doesn't have a tremendous amount of options. But this safe rate of failure isn't so for everything. Brakes on a car must be easier to understand and use than, say, the radio or air-conditioning/heating. If you misuse the brakes, or can't figure them out, it could spell your death. If you misuse the air-conditioning, you'll just be hot for a while. And chances are, you'll figure it out minutes later. 

In short, some devices might irritate us, but it's not a big deal if they do. Others however, need to make sense the instant you see it, or the consequences could be dire.