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Thanks, glad you like it! It's not actually triggered directly by FPS drops, it's triggered by desync between the midi player that plays the notes, and the music. Specially if the desync is too much, that the desync mitigation system can't handle it, which can cause the notes to start spawning inside the box, making the game unplayable. The error message is meant to explain what's happening, but maybe the trigger for it to show is a bit too sensitive. It's usually caused by/accompanied by fps drops, but it could not be if the thread the midi player is running on lags, but the main thread the game is running on doesn't.
Luckily that won't be relevant in the next update, because I've done a big rework of not spawning, so it's just tied to the music, fixing that desync!