Multiple times it happened that the edit/pencil icon (by comment) was showing but it was not possible to click on it and edit the comment.
I edited the comment I added above - you can see the icon on the end that indicates that the comment has been edited:
To edit a comment you need to mouse over the comment and wait for the
edit link to appear. You can only edit your own comments, and only for 5 minutes after you have commented. After that time you are unable to edit the comment.
edit link at the end:
If you need to edit your comment after the 5 minute timeout, you can
flag the comment and ask a moderator to edit it (with an explanation), or delete the comment and add a new one. (edit It appears you cannot flag your own comment for moderator attention)