A long time ago in a country far, far away (Australia), I was doing a PhD, and shared an office with a bunch of others, all doing various topics in quantum optics. This being Brisbane, during the summer the office was rather hot. Naturally, in the absence of any air conditioning, it was often hard to concentrate in the heat.
"Hah", I thought, I'll put a sign on the door referring to the air temperature in a way amusing to all the other graduate students. So I printed out a simple sign, saying "Warning: high levels of thermal radiation are present in this room" and put it up on the door. This indeed was possibly found to be at least slightly humorous by others, especially since in most quantum optics calculations, thermal photons can be safely ignored - optical frequencies have energies far greater than any available room-temperature thermal energies.
Shortly afterwards, for some reason that wasn't immediately obvious, our wastepaper bins stopped being emptied by the cleaners. Naturally I enquired at the Physics office as to why this might be ...