Up until recently, it was impossible to unlock Vampire Survivors MissingNo secret playable character in-game. Instead you had to edit the game's code directly. Of course, this wasn't intended by the developers, who had removed the character in one of the early updates, but now you can unlock MissingNo by playing...
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How to unlock MissingNo in Vampire Survivors
Source: PC Gamer (opens in a new tab)