Does it mean "Don't forget to knock your head here?" Obviously, it would be much less painful to bang into a light sign hanging from a chain than to bang into that horizontal bar right behind it. So I guess, in a way, they do want people to bang into the sign. It's just like those "low overhead" signs hanging outside garages -- it's better if the top of your vehicle makes that sign wiggle than if your vehicle gets wedged under a support.
What is it for? Why is it there? Did whoever put it up realize that if there were no sign, there would be no need to warn about it? Is this a joke from the developers? Is it a lesson in recursiveness? Is it a philosophical prop? --Bizarre Sign (Lushlush)
(I flipped the image and cropped it to emphasize the effect. Not something one would do as a journalist, but that sort of thing is frequently done in the context of design.)
It looks to be distinctly European. If it were American, there would be a phone number (or web URL) to place orders for more signs
Looks like another entry for the Hall of Technical Documentation Weirdness.