It would be a cool feature to be able to add a note to an issue which is (unlike a comment) shown within the issue navigator and just visible for the one who added it.

Often, reported issues are summarized with insignificant wording (e.g. "Input field does not work properly"), perhaps just because the reporter doesn't know better or how to put it.
In these cases, it would be great to have a possibility for the assignee (or any user?) to attach a small note to the issue, just as a note to himself what it really is about (e.g. "exception when inserting ampersand" or "probably encoding-related") or what he is going to do about it (e.g. "should check for special characters before processing"). If this note was shown within the issue navigator (e.g. as a tooltip - and just for the user who created it) it could be re-identified quickly on the first glimpse, and thus save the time of opening issues again, when one has forgotten what the matter was with a specific issue.