Several things plague the junior-level editor or burgeoning English major: the ubiquitous "they're/their/there" error, the "your/you're" mix-up, and eventually the distinction between stationery (letter-writing supplies) and stationary (motionless; fixed position). That little E or A tells a completely different story, yet many people live their entire lives never learning how to sort that out.
Photo: Noisy Decent Graphics
But actually, both words share the same etymological roots. In a medieval world of traveling merchants, a stationer was a man who had a fixed shop and didn't roam about: he was stationed in place. Initially his was a general store with an unthematic array of supplies, mainly dictated by demand. If people needed mandolin picks, he might cache plectrums.
In a moment of history repeating itself: 'zines (unprofessional, self-published little magazines of specialized interest) might not be enjoying the boom they once did, but in their heyday they relied on for distribution the generosity of book/magazine sellers to allow a little space to move their wares. Barnes & Noble and Waldenbooks would never go for this, so 'zine producers relied on indie shops for shelf space, and a very helpful store could earn the title of "distro," or a reliable drop-off/commission point for distributing one's 'zines.
Similarly, stationers were often hit up by chapmen, peddlers of chapbooks. The Three Musketeers was originally a chapbook: each chapter was its own episode, printed and bound and marketed as a serial drama. People would get hooked on the story, which could end with a cliffhanger (just as today's soap operas and serial dramas do: why mess with success?), and have to wait until the next chapbook was released.
This symbiotic relationship between the chapman and the stationer dictated the course of the latter's marketing, and they began to stock up bottles of ink, quills and nibs, parchment, binding thread, and other self-publishing supplies. Some even went so far as to explore offering printing services for anyone with a bug up their ass to start a little newspaper or inflammatory political document. (The word libel, "false or malicious printed material," comes from the Latin libellus, or "little book," a term meant to devalue a publication of spurious value.)
And stationery was sold by the stationer who remained stationary... so why the deviant spelling? Perhaps someone implemented an A in the latter specifically to separate the two and it caught on. Written language has always changed much more dramatically than spoken language. And there's the question of "language" versus "currently popular dialect" but I won't get into that now.