Drafts Folder in Mail.app contains messages which have been sent From time to time my Drafts folder contains an autosaved message that I haven't sent which is nice because I can pick up where I left off creating a new message, but lately most of the messages are messages that I did send. 

I then have to go and check that the message did in-fact get sent before I can delete the draft. 

This mailbox is unified from 3 IMAP accounts, one being a gmail account. I haven't seen a pattern (e.g. the zombie drafts always being from one account). 

Has anyone else fixed this issue, or at least seen it?
Gmail accounts are specifically subject to this issue.

In this Apple Support Communities post, there is a good discussion of why this occurs and how to reduce or stop it. To quote a particular response:


  Mail creates a temporary draft each time you compose a message. If you
  send the message while the IMAP account syncs, the draft doesn't get
  deleted and gets stored in gmail's draft folder.
  
  If you want to stop this, uncheck to store drafts on the server. It will put the draft
  folder back under the gmail folder and create a local draft folder for
  gmail. If you need to complete a draft on another device, simply move
  it to the IMAP draft folder.


I can verify from experience that this issue occurs in Mountain Lion as well.