List of known major vanilla bugs

Mid1 mission won't load

Map screen won't load after mid2

Monst3r cannot unlock the final terminal during the final mission

Skipping mid2

Cannot activate Security Hub or Mainframe Lock in final mission

Mods mentioned

While II is generally a very stable game, sadly Klei has stopped active support a while ago, so there are still several major bugs that remain unfixed, particularly story-related ones. For the purposes of this post, the first and second missions added mid-campaign by the DLC will be referred to as mid1 and mid2, respectively. These bugs occur on both the PC and PS4 versions of the game.Occurs when starting the game with Monst3r as one of the agents and then losing him before starting mid1. Causes an assertion error on mission load and breaks the campaign.Fixed by rescuing Monst3r before mid1. No community fix yet. If you're out of time, you can enable debug mode to spawn in a Detention Center mission even if the Omni Research Facility mission is already available. Note that if you already went to a Detention Center and could have rescued Monst3r but didn't, the bug will still occur and he won't spawn again.Occurs when pressing the 'Continue' button on the recap screen if you exit mid2 with Monst3r as your only surviving agent, and he isofficially on your team (i.e. you didn't start with him). Game will exit to main menu when attempting to load the map screen, breaking that save slot. This contradicts in-game tips/dialogue suggesting that "Our agents are expendable, but Monst3r must survive" is a viable option.Fixed by the Omni Foundry Plus mod: Monst3r will now join your agency if he is the only survivor of the mission, allowing you to continue the campaign.Occurs if Monst3r has full augment slots (four out of four potential slots filled, therefore only possible if you start with him)a full inventory when he finishes hacking the Security Hub. Normally, he gets an extra augment with the door passcodes here. If his augment slots and inventory are full, it's dropped on the ground instead and there's no way to pick it up or complete the mission.Fixed by the Sim Constructor mod. Otherwise, make sure he has empty inventory slots when he finishes the hack. If your mission has already progressed past this point, you can use debug mode to rewind back before that point.Occurs when exiting the game (by closing the application) while you're on the mission recap screen after mid1, instead of pressing the 'Continue' button to progress to mid2. Reloading the save will bring you to the map screen with the hours reset as normally after mid2, but with no new program slot and no requirement to find battery cells every mission. Aside from that, this bug is harmless.Click the 'Continue' button after mid1 and let the mission load before exiting.Occurs when capturing those mainframe devices when they have a Rubiks or Castle daemon on them. They become player-owned, but still get a firewall increase, meaning they cannot be activated by Monst3r.Hack the device with a Buster Chip, then activate it. Also fixed entirely by Function Library mod.