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It turns out that if you save one of those files containing an OfficeArt shape (a particularly kind of graphic) in Office 2003, then open it in an earlier version of Office, you may lose the whole thing. Or, in Microsoft-speak, “you may experience the following symptoms:
The document may not open completely.
The document may be corrupted.
The document may open but with missing content.
You might receive an error message.”
You’ve been warned. More details here.