When a floor is being mopped, signs appear saying, effectively, ‘if you slip, don’t sue us.’
These actually introduce a trip hazard, especially when placed at the top of stairs, for example.
Today’s invention warns people of wet floors, but creates no such trip problem.
A lightweight sign with a clamp type suction device is attached to the ceiling, instead (this could be done using eg a balloon, but would probably be too fussy and shortlived). The sign might be mostly transparent, so that collisions between passing pedestrians could be minimised.
Also, an extendable neck version might be made to help with varying ceiling heights.