"The uncertain bridge now before me was posted with a warning sign, but I took the risk and crossed again to the south bank where traces of life reappeared. Furtive, shambling creatures stared cryptically in my direction, and more normal faces eyed me coldly and curiously."
H. P. Lovecraft
Whether they are stamped out of cheap sheet-metal by the hundred according to exacting Pruztian specifications, molded from zinn by hand by the apprentices of master craftsmen in the traditional Franzikaner style, or are simply peel-and-stick decals issued by the Registrar of Transitional, Transgressive or Invasive Spaces, the yellow diamond-shaped Warning Signs are a very familiar sight to anyone who has been in Wermspittle for more than a few hours.
Scavengers, Foragers and others of their ilk post these signs wherever they discover a Weak Point, Soft Spot or similar such site of intraspatial bleedthrough. They warn of nearby Maelstroms, Vortices, Black Zones, and locations where Transitions and/or Transpositions are more likely to happen, such as around the perimeter of the Glowfield, along the fringes of Abingdon Grove, or at every known entrance to the Jumbles where Sickly Yellow Phantoms watch over the fallen monuments and rampantly overgrown Sybelline Grottoes as they perform endless series of unasked for divinations and unheeded oracles in eerie silence.
Under no circumstances should the Six Yellow Signs known to the Phantoms ever be mistaken for the Warning Signs designating dangerous, strange spaces. Yes, they are yellow, but that is the fault of some nameless faceless bureaucrat and despite the best efforts of well-meaning civic-minded mobs and delegations this has not changed in over six hundred years, nor is it likely to ever change.
The Warning Signs placed about the Inner Ramparts are all much more sturdily-constructed than the civilian versions found elsewhere, in order to withstand frequent exposure to Black Smoke and other forms of eldritch ordnance.