[Update]
I was looking at the renaming options section and completely missed Paragraph 4. Sorry =P
When someone renames their journal and chooses the "do not redirect" option, old comments they had made will bear their new usernames. This looked like a bug to me until 
supersat explained the design flaw.
In that case, I think a little further explanation should be added to FAQ #25 to reflect this so that someone doesn't pay $15 thinking that the rename is completely foolproof.
All it says right now is:
Alternately, the old username can be marked as deleted,
so anyone who visits the journal will receive an error
message. The old username will remain unavailable for
at least 30 days.
That gives me the impression that under all circumstances, the old username will look and act like a deleted journal. Which is not the case with comment headers.