Problems saving draft mailings

Someone reported: There have been several instances of CiviMail not auto saving and presenting an error popup. It basically says something happened and it couldn't save.

I briefly saw the problem but not able to say exactly when it happens. The one time it happened I think the changes were actually saved, not sure.

While testing I see that deleting the subject line and saving causes problems.

To replicate: Create a new mailing, add a subject line and some text. Save and open it again, change the text a little and delete the subject. Save the draft. (Do not wait for autosave)

On reopening the draft, you'll see the changes are lost. This was replicated on Demo.

There's a StackExchange question that may be related to this: http://civicrm.stackexchange.com/questions/7762/civimail-not-saving-changes-to-email-drafts