When you are being all super-efficient and shoot off a quick email with the sole purpose of forwarding an attachment. And you give said email a short title and body that says little more than 'here's the attachment mate'. And you then proceed to press send on the email but have forgotten to attach the actual attachment.
I sent a phantomachment to Larry yesterday, it was meant to have the business records on it but now I look like a fool.
Oh crap, I just FTA'd the poster for that email. It means simply Forgot the Attachment. It could be FTDA if it was really important or really dumb.