Ganymede

Henceforth no man shall set foot upon the world, and all around shall be set sentinels to ward away unwary spacecraft. We must accept that this place is lost to us forever, and is now the eternal habitation of abomination.
-- from The Contagion of Ganymede

The largest moon of Jupiter and formerly a Forge World. The Adeptus Mechanicus infamously used this world as a test bed for stolen Squat technology, specifically their "Warp Fission" technology, wherein the Squats would use raw warp energy and process it as a form of power.

The Squats, being extremely more advanced than the AdMech, who have religious ceremonies to reboot the vending machines in the Forge World Lobbies after they eat your credits, are able to handle this technology. The Imperium is not.

The resulting breach caused a warp rift -- a miniature eye of terror -- to form on the planet's surface, and it has been declared off limits for all time.