"stationary" vs. "stationery"

There’s no spelling error here. Although both words are pronounced the same way,  “stationary” and “stationery” do not have the same meaning. Yet, you sometimes see mistakes.

• stationary (adj.) – not moving
  
• stationery* (n.) – writing materials like pen, pencil, eraser, exercise book etc.

Therefore, it is incorrect to say:

• I want to go to the bookstore to get some stationary. (X)
• Vehicles were stationery for hours on the highway. (X)

*NOTE:
“Stationery” does not have a plural form, so you cannot add an “-s“. So, it is wrong to say:

• I’m buying some stationeries. (X)
• I’m buying some stationery. (√)