I have a customer I upgraded to 8.5 and some of their users have different unread marks on the two clustered servers (unread marks are set to “replicate to all clustered servers”).Since the unread marks are per user, when I do this, I see matching unread counts for all folders on both servers 
Subject: Yes, but not in a Cluster
My mail-file ist on two different servers and locally. The unread marks on my local file are never correct. Thus I’m working always (@home or @work) with the file on one of the servers.
Subject: Never seen them work properly
I’ve never seen unread marks work 100% at all times on any Notes infrastructure.
I laugh thinking of the “Are unread marks fixed in R5” technote that was written in the late 90s.
Subject: yep, that technote is ironic
From what I understood, it was a big redesign of the whole unread note tracking mechanism.
Worked around it for now at this customer by making sure the unread marks are set to “clustered”, then creating a new replica of all the mail DBs on the secondary clustered machine. Seems ok for now…until one of the machines goes down for a little while no doubt
Subject: Most frequent complaint
In nearly a decade of running the Notes system here at (insert big car company name here), I don’t hesistate to say that unreads marks are the single biggest tech support issue.
As they don’t work and there is no magic fix, the tickets always gets escalated to Tier 3 support where we have to explain time and time again that it “just doesn’t work”.