User blog comment:SakuraKeyblader/Effin Wall of Flesh!/@comment-4674838-20111227035904

I think the current system is a little rough on casual gamers. Having the Wall of Flesh appear after a voodoo doll accidentally falls in lava isn't entirely unreasonable... if the player is aware that that can happen. If they know about that ahead of time, players can simply not kill voodoo demons when they're flying over lava--either by luring them above solid land, or by building some kind of platform over the lava.

One solution that I think would work is to just have a little message appear on the screen saying "A voodoo demon is approaching..." every time a voodoo demon spawns. This mirrors how messages appear when shadow orbs are smashed and how the Eater of Worlds spawns after destroying 3 of them.