Superheroes not allowed?

Don’t try to climb vertical surfaces!
Originally uploaded by tommyajohansson

I’m really puzzled by this sign.

My spontaneous interpretation is don’t run up this wall, but given that only superheroes such as Spiderman can do that, I guess it must be outlawing them instead.

I guess in theory the photographer might also have rotated the picture 90 degrees before uploading it, but then it just gets even more bizarre – something like don’t dance wildly?

Has anybody here got an idea what it might mean?