I agree with Rahul. If you're seeing this under certain conditions, then the technote hits the mark (and I worked on the associated Etrack). If you're seeing it outside of the following sort of condition, then it's worth investigating further ..
Conditions you'll see this under :-
Highlight 100 unread items
Right click, mark them as read
Click the Store in Vault button in Outlook
*Some* of the items won't stay read, they'll flip back to unread.
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