Exchange 2010 - single server, SP2.
Some users are sometimes having issues changing or declining meeting requests - most often when the affected user is *not* the organizer.
On Tuesday, User1 scheduled a meeting for Friday and invited User2.
User2 accepted on Tuesday.
On Friday, before the meeting time, User2 proposed a new time, then declined. (I witnessed User2 do this - so I know for a fact he did it when he said he did).
User1 did not get the change and cancel emails until 2 hours later (well after the meeting, and some confusion).
Message tracking shows the emails to User1 only at the 2-hours-late time. Nothing was sent when I saw User2 propose the new time and then cancel.
This is one example, but not the only condition. I have not been able to replicate it on demand.