I/O stands for input/output. The reason why this happens when you try to download a project while it's loading is because the Flash player is unable to access it while you're trying to download it, I think. I could be totally wrong, however.
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It also happens just before you navigate off a page while it's loading. Not a major bug though.
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Okay yeah so it's just that it can't read the file properly.
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What everyone said
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When you click something on the page like the download button or the thumbnail of another project, that's what comes up a lot for me.
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