Affects Version/s: Minecraft 1.12.2, Minecraft 18w02a, Minecraft 18w03b
Fix Version/s: None
If a block is placed inside the lava, instead of every single block of lava within view distance instantly being transformed into obsidian, only a single strip along a single axis in a single direction is transformed.
When the offending block is removed, blocks solidify on this same axis, but going in the opposite direction.
If the block is replaced, a new strip hardens in the same direction as the first, going in the exact same direction, and removing this block flanks it on the other side.
Removing any of the obsidian blocks seems to crash the game.