The Shadow Weave Mask ( http://db.vanillagaming.org/?item=10025 ) could be learned by the tailoring trainer in UC and SW ( http://db.vanillagaming.org/?npc=4578 UC - http://db.vanillagaming.org/?npc=9584 SW ).
Or, blizzlikely speaking, to get this pattern, you must complete a quest, 'The Undermarket' ( http://db.vanillagaming.org/?quest=3402 ). This is the only way to get this pattern.
http://us.battle.net/wow/en/item/10025#tab=comments
This mask is no longer in the game currently, as the NPC who gave the quest to learn it got removed with Cataclysm.
http://eu.battle.net/wow/en/forum/topic/6444085740
This item was a quest reward for a quest available only to tailors. What's curious about it is that despite it being a quest reward, it wasn't BoP, so theoretically a tailor could complete the quest, not learn the pattern, and put it up on the Auction House to sell it. Of course the pattern could only be learnt by other tailors, so a tailor would likely only buy it in the event that they hadn't yet completed the quest.
Seems like it's the same kind of Mangos bug as the LOS in battleground.
