One of my recent responses in Stack Overflow includes a comment that just says "commented on" with no question title, and there is no link to the question, so I can't click on it to see where it's coming from.
This seems like a bug. (is it a deleted question?)
The particular comment is by Idigas and the comment is "@S.Lott - nice one.", but I have no idea which question so I can't link to it for you.