The stationery / stationary pair mix-up is one I see a lot. It’s an easy one to get wrong, but hopefully an easy one to get right (or else just bookmark all these troublesome pair posts, I suppose!). If it comes to it, learn “stationery” as I’m fairly sure I see that more often than “stationary”, on the grounds that if you know one of them, you can work out the other one. Or something. Anyway, here we go:
Stationary – not moving, still. “The traffic was stationary”
Stationery – writing materials. “My wedding stationery has a pink theme”; “Have you done the office stationery order yet?”
“The stationery truck is caught in stationary traffic and we have no more notepads until it gets here!”
You can find more troublesome pairs here.