Guerilla Transport for London stickers with spoof notices for passengers are being plastered on Tube trains and stations.
The notices, in the same colour and font as genuine TfL signs, include "No eye contact. Penalty £200" and "We apologise for any incontinence caused during these engineering works".
Others suggest commuters should offer their seats to "drunks less able to stand" and advise: "Peak hours may necessitate you let other people sit on your lap."
A spokesman told the BBC: "It's a form of rebellion, whether it be due to the current climate of doom and gloom or people wanting to brighten their day."
The stickers, apparently designed to brighten commuters' daily journeys on the London Undergound, are bring celebrated on a Facebook page.
However, a spokesman for the British Transport Police branded the stickers "unwanted vandalism that causes criminal damage".