Spread Poisonous Goo In Dalaran, Other Capital Cities (Patreon)

The boss Viscidus from Ahn'Qiraj has an ability called "Poison Shock". If you port somewhere during the debuff this effect stays with you, leaving a pool of goo on the ground in Dalaran/wherever. 

This is what it looks like: http://i.imgur.com/jhedmC5.jpg

The ability won't do significant damage to high-level players but keeps players in combat. It appears to persist indefinitely, perhaps till manually fixed by Blizzard or reset.