ReBoot/Nightmare Fuel


 * The Medusa Bug (from the eponymous episode in the first season) spawned a city devouring plague that threatened to petrify a city and doom its residents to death by energy decay. The fact that nothing could stop it made it scary, the fact that there was nowhere to run made it terrifying, and seeing the woman who unleashed it laughing as it consumed Mainframe made it even worse.
 * At least Bob was immune & got Hexodecimal to reverse it w/a Victoryisboring speech. He actually congratulated her on bringing "peace & quiet" to Mainframe. She let out a Big No upon the realization that w/ everything petrified there would be no audience for her mad antics.
 * The end of "Game Over", in which Enzo's Beginner's Luck finally runs out, leaving Mainframe without a Guardian and everyone presuming him (and AndrAIa and Frisket) nullified. Beware of everything following the computer's announcement of "Game Over".
 * End Prog. Its the end of the world and there's nothing you can do to stop it. Everything falls apart as the system voice says "Warning, System Crash" over and over with an alarm noise constantly going off. Everyone dies as everything fades to black.