In a similar vein to
but definitely less important...
I recently tried to post a comment and had it go through without the "@[nameofanotheruser]" I had typed at the beginning (I was addressing another user.) I ended up trying multiple times to edit the comment to add that back in, but it would never 'take'.
I thought the system was just not working, and the problem was exacerbated by the fact that the other user had a name with non-standard characters (so I thought that may have been causing the problem).
After I asked about this in the comments, that user clued me in to the fact that @[nameofanotheruser] is not required (and for some reason is actually forbidden...not just pointless) when you are posting a comment on that user's answer, but this could have been explained more easily with a simple pop-up message delivered at the time the comment is being entered.
The pop-up text could be "'@[nameofanotheruser]' is not required here. Users are notified of all comments on their own questions and answers."
Alternatively, those "@" statements could just be allowed is those cases. They may be redundant, but unless they actually hurt something (double notification, maybe?) stripping them out may not be worth the confusion it generates.
Edit:
More support for the 2nd solution I mentioned (just allow @ statements in all cases):
Even under [anotheruser]'s question or answer, @[anotheruser] can be even more than just harmless, it can still serve a purpose! It still conveys to that the comment is directed at [anotheruser] and not the wider readership. ...in addition to simply keeping the look of directed comments standard, and preserving WYSIWYG.