I sent my obsidian farm to nega-space.
My only theory as to how involves a bit of a burrows snafu, where I told the miners to mine the obsidian but didn't set the adjacent spaces as inside the burrow, so they went there and had nowhere to stand. So I included the surrounding space in the burrow. Didn't help.
So I had them channel the obsidian away from above. No problem, it's part of the obsidian farm system reset process anyway. But the Dwarves wouldn't dump the obsidian rocks into the quantum stockpile. It wouldn't even generate Dump Item jobs. So I made a stockpile of nothing but obsidian. The Dwarves still wouldn't touch the cursed obsidian. So I tried to build a wall inside the obsidian farm, right on top of one of the stones so that the pathing couldn't possibly be messed up.
I was very, very, very wrong about the pathing not being messed up.The distance from the wall to the obsidian in the same space?
Negative 20,000 spaces.
The distance from a tile to the obsidian in an adjacent tile: negative 12,500 spaces. Usually. It's actually random out of -10,000, -12,500, and -20,000. And in some parts of the room it won't let me try to build anything because the distances to all available materials are apparently too hugely negative.
Okay, thought I, I'll just save and reload the game. Maybe the pathing will reset into something resembling coherence.
WRONG, BITCH! ...says the game.
I tried to build a wall in there. "there is no access to non-economic building material." Oops, I'd set all of the obsidians to be dumped. Silly me. But the obsidian farm is apparently still in nega-space, since that area should have access to all the stone in my fort.
Try to build a wall one space away from an obsidian stone. Distance: -1.
Try to build a wall on top of an obsidian stone. Distance: 0. Promising.
Try to build a wall in a different spot (right on a stone). Distance: -1,500.
Try to build a wall in a different spot (1 away from a stone). Distance: -4,000.
Try to build a wall in a different spot (right on a stone). Distance: -8,500.
Try to build a wall in the same spot. Distance: -12,500.
Try to build a wall in the same spot. Distance: -16,000.
Try to build a wall in the same spot. Distance: -17,500.
Oh, and just to top this whole mess off with a space-nullifying cherry, it says that there are 31 units of obsidian available to build stuff in this space with. So I opened up the specific building material sub-menu. One at distance 0, two at distance -26,500, and that's it. After a quick poke around, 21 apparently means 2, 31 apparently means 3, and 41 apparently means 4. Oh yay, a shred of consistency in my garden of entropy.
What the epic fuck.