Ok, so there is a difference between these two words and it'd be lovely if we could all remember it. If we can't, here is a little reminder for us:
Stationary: [adjective] not moving or not intended to be moved
Where you stand still or something doesn't move.
Stationery: [mass noun] for writing and other office materials
To be found in a store where you can buy pretty paper and such.
Clear now? Excellent!