Most naturally generated walls are considered unsafe. Walls that are converted via biome spread or the Clentaminator / Terraformer are also considered unsafe, even if they were previously safe. Very few items allow placing unsafe walls by the player.

The Hallowed Prism Wall, Hallowed Cavern Wall, Hallowed Shard Wall, and Hallowed Crystalline Wall are types of background walls which can be found naturally generated in the Underground Hallow, considered unsafe. They do not drop as an item when destroyed, and can only be obtained on the Desktop, Console, and Mobile, via crafting.