If the word "attached" is entered into a message being composed, a message pops up at the bottom of the window saying "The message you have composed seems to refer to an attached file but you have not attached anything. Do you want to attach a file to your message?" This happens regardless of whether or not in Settings / Configure Kmail / Composer / Attachments tab, "Enable detection of missing attachments" is ticked. (I've never seen the message before 4.13. Yet there's a blog post from 2004 where someone comments on it. Strange.)
I confirm it.
Git commit e41515897c2701fec9892259735c2b81f55fa518 by Montel Laurent. Committed on 23/04/2014 at 19:15. Pushed by mlaurent into branch 'KDE/4.13'. Fix Bug 333778 - Disabling "Enable detection of missing attachments" setting ignored FIXED-IN: 4.13.1 M +5 -3 kmail/editor/kmcomposewin.cpp http://commits.kde.org/kdepim/e41515897c2701fec9892259735c2b81f55fa518