Unwinnable by Mistake/Web Original

This is what happens in the Bad Future of Homestuck. John gets himself killed by fighting his Denizen far earlier than he's supposed to. Without him, Jade (presumably) dies since she is unable to enter the Medium and escape from the meteor that later strikes her house. Dave and Rose can't progress past a certain point without their help, and Dave is forced to travel back in time to Set Right What Once Went Wrong.
- The whole kids' session of Sburb could be considered this too, at least as far as winning it normally is concerned, because they accidentally prototyped Becquerel and gave all of the enemies in their game ridiculous game breaking godlike powers that broke the game so badly that one of their bosses escaped their universe and messed the trolls' session up - sort've like failing on your video game so hard that someone else in a different city's game disk scratches. This is compounded by the fact that Jack started the Reckoning way, way early and the kids game basically started just as they ran out of time for some reason. The characters involved realise something went wrong and are trying to figure out how to salvage the situation.
- Homestuck's Sburb is confusing in this regard. Because it surpasses universes, the kids' session wasn't unwinnable, technically, any more than Final Fantasy 6 is unwinnable because the World of Balance got destroyed. The game accounted for their apparent "errors" and they are no proceeding to their equivalent of World of Ruin.