Mario Paint only has 32KB of save RAM, which isn't enough to fit an uncompressed copy of both the regular canvas and the animation canvas. Because of that, I wondered what would happen if the two canvases were so complicated that it couldn't compress them small enough, and I guess the answer is this. If you want to replicate this yourself, other people have had success getting enough entropy by just using the spray tool and using all of the colors (but remember that you need to fill the animation frames too, not just the main canvas).
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