That error comes up when an old version of SCRATCH tries to run a newer project (such as SCRATCH 1.2 trying to run a project designed in 1.3) What version of SCRATCH are you running?
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It also happens when you download a project with Firefox and use "Open" rather than "Save as". This was fixed in version 1.3.1 - If you have a older version, please update it here: http://info.scratch.mit.edu/Scratch_1.3.1_Download
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