7. Unleashing The Metalheads Upon The World - Jak And Daxter: The Precursor Legacy
Whilst this event is not seen until Jak II: Renegade, it's the actions of Jak and Daxter in their first outing that set up the disaster they bring about. Once the player has collected 100 power cells (all but one in the entire game), they gain access to what is revealed to be a rift gate in the second game. Upon first opening the portal, they are greeted by swarms of robotic alien locusts and are thrust into a dystopian police state known as Haven City, ruled over by the despotic Baron Praxis.
It is revealed later on in the game that Haven is built upon the ruins of Jak and Daxter's world, and it is all the result of centuries of conflict caused by the heroes opening that rift gate and making way for the Metalhead invasion. Worse still, when the characters meet a younger version of Samos the Sage, and an infant child who is revealed to be Jak himself as a child, they then use the rift gate to send their younger selves back to before the events of the first game, therefore creating a stable time-loop in which the world is doomed to forever be overrun and conquered.
Weirdly, the game fails to acknowledge that this is an inherently bad thing. Because we were still in the mascot era of gaming, we were treated to fireworks and a celebration rather than a chastisement for ruining the world forever.