Soulstorm. Or as I like to call it, "Shit".
On release, the game was very well-received - and then, it happened. 19 hours after the game's initial launch, an infinite resource exploit was uncovered for the Sisters of Battle. By queuing up an upgrade for their listening post and then cancelling it, it was possible to get double the normal resource amount - ergo allowing you to do it rapid-fire for colossal amount of resources very, very quickly along with having infinite faith resource by simply auto-casting a faith ability so in-theory, a simple Celestian squad with a missionary attached could very well turn their durability like those of an Assault Terminator Squad with a Chaplain attached. After initially dismissing this as "not a real bug" and prattling on that players "should be happy with what they've got," a "hotfix" was promised within "one, maybe two weeks."
That one to two weeks turned into a nearly 9-month wait, whilst the game suffered horrendously. The "merit" system which was designed to award players with little collections of in-game multiplayer achievements was completely non-functional not only during this time, but after it. The lobby at launch read: "This is stand-in-news. Replace this with real news," and kept this for 6 our of the 8.65 months it took for the patch to hit. During this time, Automatch was broken and the use of a trainer allowing players to play multiple races and delete enemy buildings ran rampant, destroying anything that remotely remained of the game's competitive multiplayer environment. Smaller bugs and problems kept popping up during the wait: using dance of death would set Eldar players' resources to Zero (The fuckers entirely deserved it, though). Charming an Ethereal with a Deceiver would give the Necron Army billions of hit points per unit. Observers could activate a Dark Eldar player's Soul Powers.
After 8.65 months, there was nothing left. The game was deader than a Hooker at Matt Ward's place. But again, after 8 months of waiting and leaving the game to die, a small shard of hope appeared: the hotfix finally DID arrive which addressed most of the early problems present in the game, including the ability to use trainers in a multiplayer game and most of the SoB/DE bugs, which made the game relatively playable. However, much like how Zelda to the Nintendo Gamecube, it came too late and Soulstorm basically died out as many players just gave up on the game.
(Thank Slaanesh for 1d4chan!)