User A has granted User B access to view/manage her calendar.
User A received an email from another person and copied it into the calendar.
User B is unable to view this calendar entry and made a calendar entry for the same date & time for a different meeting.
User A is not happy - cannot be in two places at once! 
Why was User B not able to see the calendar entry that User A had made?
Subject: Calendar issues
I am having issues with calendar too. User’s schedules are not displaying properly for others that are trying to send them invites to meetings.
The user will accept the meetings, and won’t know they have other meetings in that same time slot.
I sent this to IBM developers and waiting for an update on this issue.
I am running an 8.5 domino server with 8.5 clients and a handful of 8.5.1 clients. One being myself.
Subject: Thoughts…
If the new entry UserA created is not a public document then UserB will not be able to see it if they open UserA’s Calendar. That is if UserA clicked on “Mark Private” then only they will be able to see it.
Also, is the entry in the replica that UserB is looking at? I know of some cases like this where UserA is using a local replica but not always pushing data up to the server before UserB goes in and does something similar (typical AA/boss scenario). So unless both are in the same replica that could be another reason.
Bruce
IBM