Talk:Wood Platform/@comment-5062819-20120429105727/@comment-1863391-20120430112050

As long as there's any water at all occupying the same tile as the Platform, Obsidian will not form.

Two tiles (Obsidian and Wooden Platforms) can't occupy the same place, and the game's engine isn't told what to do when Water and Lava mix in a situation like that, so they just sit on top of each other..