I just recently found out that if you delete a sprite (always works with the Scratch Cat that appears on startup) and then immediately press the "Paint New Sprite" button, the sprite that you create won't show up in the list of sprites and you can only edit it by double clicking on it. I also found that this is more susceptible with larger sprites since they take a bit longer to erase from the project.
This glitch doesn't seem to effect the Scratch operations in any way, but I just thought people would like to know that it's there.
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Hmm, I think you've found another method of activating the hidden sprite trick...
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