Life Tap

Life Tap is an instant-cast warlock spell that trades a little bit of health for a huge chunk of mana. This spell was ridiculously overpowered during TBC, where warlocks in full PvP gear and over 15k hp would Life Tap for 10% of their HP and get 50% mana back.

At some point during the 2.4 PTR, the spell was changed to work based on % amounts. This caused a massive outrage from the warlock community, whose tears flooded the official forums as well as Blizzard's headquarters, causing the change to be reverted on the PTR before going live.