Last modified: 2014-09-11 23:41:19 UTC
Horizontal lines (----, <hr>) are extremely hard to delete. Unlike other non-editable things one might want to kill in the article they're only one pixel high andare hard to click on or otherwise select.
I ran across this just now. Not only was I unable to find a way to delete it, I'm not sure that I'd have known that I had successfully selected it.