Vending machine* usability

* I grew up in the South, which means I actually call these "Coke machines," but in the interest of generality I'll refrain from that in this instance.

I bought a soda recently, and noticed (for the first time) a pretty basic design flaw that is common - though not universal - in vending machines. You know how these machines have a picture of a dollar bill to show you the correct orientation to insert it into the machine? Well, some machines will only accept bills in that orientation, others will accept bills face-up regardless of which short edge is inserted first, and still others will accept any bill regardless of orientation. I'm assuming that the machine is set up to prefer one orientation over another so that it can compare features of the bill to a template - a search image. If that's the case, though, it should take minimal effort to allow any of the four orientations; you'd just have to add three more search images and identify the relevant features. Of course, it's not quite that simple, since the design of the bill changes over time - but those very design changes show that this is possible, since a vending machine may have to respond to several different bills even if they're all inserted in the same way. Far be it from me to speculate on why vending machine manufacturers haven't done this universally - maybe I'm way off base in my assessment, and the challenge in accepting bills any old way is in fact very difficult - but it's just another of those minor usability failures that add up in the course of a day.