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Microsoft Office 2008 applications, especially Word, do not work well with the Spaces feature in Leopard. This is not a newly discovered bug; but it is one that is still with us.
Microsoft appears to place the blame on Apple, stating that the symptoms are "caused by the architectural implementation of Spaces in Mac OS X v10.5 (Leopard). Apple is aware of this issue." Neither Microsoft nor Apple have provided a fix over the course of numerous updates to both Office and Mac OS X 10.5. So it looks like this bug may remain with us for a good while longer.
Symptoms of this bug include: an inability to bring Word document windows to the foreground by clicking them, an inability to assign two Word documents to separate spaces, and plain old crashes of Office applications.
The work-around for now is to not use Spaces if you plan on using Office 2008. Or at least don't start switching among different spaces while Office 2008 applications are open.
Read article at Macworld.com