When someone comments and closes/reopens a ticket, two events are created on the timeline.
The timeline in 4.0 should be rewritten to allow grouped timeline events. This will also help with mass actions.
So instead of multiple events, it would look something like:
Sunday, 24th March 2013 11:27PM Enhancement #123 (Better things) closed as Completed Commented on ticket Better things Defect #234 (It's broked) closed as Fixed Feature Request #345 (Cool stuff) closed as Completed by Someone