Pearlstone Wall

The version that generates when the hallow overruns a chasm counts as unsafe, so enemies keep spawning in front of it and it pushes the conversion into the stone walls around it. The one you craft and place yourself does neither. Same sprite, opposite behavior, and it trips up people who tear a hallowed chasm down and rebuild with the drops expecting the spread to continue.

Holy Water lands in a much tighter radius than a Clentaminator burst, which paints a wide column and takes the blocks with it. If all you wanted was the background of one room changed, thrown flasks let you creep the edge forward a few tiles at a time instead of repainting half the map and undoing it later.