There appears to be no pattern to who is receiving the messages by mistake, or who the glitch affects.
ZDNet has confirmed first-hand that the glitch is present within Google Apps for Business accounts, including those that have not yet switched over to Google's new Hangouts platform. Messages appear to be visible on the mobile version of Hangouts. There is no indication that the messages are being received by a third party.
When contacted for comment, a Google spokesperson told ZDNet: "We're investigating reports of an issue with Google Talk. We're sorry for the inconvenience. For further updates please refer to the Apps Status Dashboard."