Doki Doki Literature Club! features variables named anticheat in order to prevent a player from loading saves they aren't supposed to, such as those from earlier acts or from other players.
The game stores a save-irrespective persistent anticheat value, and a save stores a separate anticheat variable. These are compared to check for cheating. To ensure that a player doesn't accidentally trigger the anti-cheat event, save files are deleted whenever the anticheat variable is updated.
Attempting to Cheat
If a save is loaded which contains an anticheat value differing from the persistent value, the game will say "The save file could not be loaded." followed by "Are you trying to cheat?" Monika will proceed to appear, saying "You're so funny, [player]." If the player has not yet set a name, Monika will simply say "You're so funny." The game will then perform an utter restart, forcing the player to play the game again from Act 1.
Anticheat Manipulation Points
At the end of Chapter 6, when the main character walks into Sayori's room to find her dead, the persistent anticheat value is set to a random number between 100000 and 999998. This ensures that a player who keeps save files outside of the game folder will be unable to load them into the game after restoring them.
After the short glitched scene which plays at the beginning, the persistent anticheat value is set to a new random number, and the save's anticheat value is updated to match it. This ensures that a player may only save and reload following the end of this scene.
At the end of Act 2, the persistent anticheat value is set to a random number, ensuring that no externally saved files can be loaded when Act 3 begins.
The final manipulation of the anticheat value occurs at the end of Act 3, once again ensuring that a player may not load a save to return to Act 3. However, the save-specific anticheat value is not updated, meaning that loading a save appears to be impossible in Act 4.