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."
Update: Office Update You Should Probably Have
If you've already upgraded to Microsoft Office 2003 (why, exactly?) there's an update you should download. This update, Microsoft says in its understated way, "fixes a problem that occurs when you try to open or to save a Microsoft Office PowerPoint 2003 file, a Microsoft Office Word 2003 file, or a Microsoft Office Excel 2003 file that includes an OfficeArt shape that was previously modified and saved in an earlier version of Microsoft Office."
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:
You've been warned. More details here.