Did you ever forgot to attach a file to an email you just wrote - similar to the following:
"Dear Bill, please find attached my combined patches to your Vista Kernel. I fixed 975.765 critical and 658.230 fatal errors."
You found the mail appealing, pressed the "send" button - and desaster stroke: Bill received the mail, but without the attachment. The product was delivered without your valuable contribution :-)
Stefan was really nice - he attached the screenshot of a fancy mail.app plugin names attachment scanner (oh - did I mention this is for Mac-users only?), that checks the wording of your messages for hints of attachments - and warns you if you send the mail without the precious contents.