When running Outlook in online mode, if I click on the Categories column heading to sort by Categories and then click it again to sort it in the ascending order, the items in the folder disappear and Outlook states "There are no items to show in this view."
This only happens in online mode, no problems when using cached mode, and it only happens with mailboxes that are on Exchange 2010. If I connect the same client to a mailbox hosted on Exchange 2007 I don't have the problem.
The servers are running Exchange 2010 RU4 and I can reproduce this with Outlook 2010 and 2007 (haven't tried 2003).