If I'm sending email and I've used the word "attached" in the message, it's likely that I intend to attach a file before I send. Since in many mail clients (for instance, in Mutt) the process of editing text is separated from the menu where you can attach files, I sometimes forget to actually attach the file before sending the message. If I've said I'm going to attach a file and I try to send the message without doing so, my mail client should warn me.
Internationalising this might prove to be an interesting challenge.
post-mode implements this for Emacs, and KMail has done it for ages according to a correspondant, so I think this idea's been thoroughly done now...