sometimes it's the little things
I was looking through the ways you can tweak gMail and found this. I can't say how often it would have prevented me from looking like an idiot.
Forgotten Attachment Detector - Prevents you from accidentally sending messages without the relevant attachments. Prompts you if you mention attaching a file, but forgot to do so.
Forgotten Attachment Detector - Prevents you from accidentally sending messages without the relevant attachments. Prompts you if you mention attaching a file, but forgot to do so.