http://pastebin.com/bWq5FyhB

Have a good day :) You can tell at me in IRC

Me blames vanilla structure code for being obfuscated and too annoying to figure out. Sorry, this has been a rare issue for about a year I guess, got no idea what's causing it, and it disappears when you reload the game. HEE 2 will have fully custom generators that won't have this issue. Got to mark as invalid, because I can't reliably replicate it either :/