There’s a great comment thread on designing credit-card readers to be more obvious, over at uiandus.com.
And this one, from Chris Clark, sounded the most cost-effective and simple to me,
An idea: put the scanning mechanism into the main body of the machine (in this case, the left) and give the inactive side of the swiping channel a very low profile.
The channel should be deep enough that your card doesn’t spill out during a swipe, but shallow enough that you can see that the magnetic strip won’t be touching anything if you slide your card with the strip facing ‘out.’
If people work with the assumption that the magnetic strip must touch something to work, this design removes the perceived affordance of the ‘wrong’ side.
You could get the same effect by using clear plastic on just the “short” side. But I prefer Chris’ concept, because clear plastic will get dirty, scratched, and opaque, but empty space will stay empty.
There are lots of great comments, and I don’t know enough about building these things to know which plan would give the most bang for the buck in reality. So if this problem interests you read the blog and pick a winner for yourself.
(UPDATED 2009-02-12: I wanted to clarify why I’m ignoring the most obvious and right answer, of having a sensor in each side of the machine, so there wouldn’t be a wrong way to swipe the card. My understanding is that doing that would be too costly. If that isn’t the case, then I’m deeply disappointed in every credit-card reader I’ve used, and the cheap bastards who opted to save a few bucks to inconvenience all their customers.)