Our office space is severely limited and at times doorways have been blocked with cubicle walls or filing cabinets. Every so often someone will try to open the door either banging the blocking furniture or the person discovers there's no way in. Some bright person tried to solve this by applying a little sticky note sign:

"This is not a door"

I so badly want to add others including "And this is not a door knob"
"And these are not hinges", and "And this is not a door frame"

I'd also like to tack on "Brought to you by the stealth research program"