Unless you've got an Addin you haven't told us about that's interacting with your table copy/paste activity, the only other thing I can think of that might be playing a role here is your Word installation's copy/paste settings (File|Options|Advanced>Cut, copy and paste), but even that I would only expect to play a role if you're copying between documents and the Table Style definitions conflict. My Word installation's copy/paste settings are all set to Word's defaults.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]