I have found a bug involving the "Comments may only be edited for 5 minutes" message.
- After writing a comment, stay on the page until the comment is 5 minutes old.
- Click the (still visible) "Edit" link.
- Make a change to the comment and attempt to save the comment.
- Note that the "Comments may only be edited for 5 minutes" message appears.
- Click the "Cancel" link.
- Click the (still visible) "Edit" link.
What happens is that the text in the edit box now has xComments may only be edited for 5 minutes appended to it several times.
Not sure if the solution is fixing the appended text or hiding the "Edit" link, but regardless, this is a bug that occurs on Stack Exchange sites.