The death of The Sleeper - EverQuest
"The Sleeper" was a creature found in Everquest that was intended to be unkillable. However, instead of making it non-targetable, SOE simply gave it so many hitpoints that in most cases it would have been impossible to kill. Of course, they never expected that 180 players from three separate guilds would come together to try and take down the creature.
After taking almost 4 hours of fighting to down the beast, what was their reward? SOE despawned the creature stating it was "never intended to be a part of the story". The amount of backlash against Sony for this act caused many to lose faith in SOE's ability to manage the game properly.