STEPS:

1. Go to a file page, like F1714297
2. Add a comment and save it.
3. Realize all your typos.
4. Click "Edit comment" from the comment dropdown.
5. Be confused that your browser (FF 47) still displays that dropdown that you just clicked.
6. Edit your comment and click "Save Changes".

OUTCOME:
on this instance:
Unhandled Exception ("AphrontDuplicateKeyQueryException")
#1062: Duplicate entry 'PHID-USER-.....-PHID-XACT-FILE-.....' for key 'key_draft'
or on our instance:
Get a HTTP 500 error for your POST. Nothing happens in the UI.