http://wowwiki.wikia.com/wiki/Combat
Some classes have abilities that remove them from combat instantly. However, if they are dotted they will usually be brought back into combat the next time they take damage. Note that these abilities no longer remove you from combat during Boss Fights (only at the end of wipes if you time it correctly).
http://wowwiki.wikia.com/wiki/Shadowmeld
Wrath-Logo-Small Patch 3.0.8 (2009-01-20): This ability will now properly restore threat when it is canceled. In addition, it will correctly remove a player from combat if it is active when that player is the last person left alive that the creature hates. This ability is now also unusable while affected by Faerie Fire.
Wrath-Logo-Small Patch 3.0.3 (04-Nov-2008): Cooldown starts upon melding instead of on becoming visible.