Yesterday I saw a sign in a shop that said “Stationary” attached to a shelf. 

They were absolutely right: that shelf wasn’t going anywhere. I suspect it’s still in the same place even now, although it’s been about 30 hours since I was there. 

This is a common mistake because people often don’t realise that ‘stationary’ and ‘stationery’ are two different words.  They sound the same, but are spelt differently and have very different meanings.

Stationary means “not moving”. 
A train stops at a station, and remains stationary while people get on and off the train. 

Stationery, on the other hand, is the sort of supplies you’d get at a Stationer’s stop: paper, pens, pencils, erasers, notebooks, and the like. 

Therefore, in order for the sign in the store to have been fully accurate, it could have said “This stationery shelf is stationary”. 

I suspect, however, that most people  would be less appreciative of such a sign than I would be.