Water can turn lava into obsidian from underneath.
I had a stone (not cobblestone) generator which is lava on top and flowing water below it but then what happened was the flowing water somehow turned the lava into obsidian from underneath.
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Added affects versions: 0.11.0 Beta 14
Confirmation Status: Unconfirmed → Confirmed
Added Assignee: Daniel Wustenhoff
Resolution: Unresolved → Fixed
Added affects versions: 0.12.1 Pre-release

Confirmed on beta 13 & 14