Google's Hangouts transition from Gtalk hasn't been perfectly smooth so far, starting with the fact that it upset many people because they couldn't properly see who's online and who isn't anymore (which they recently fixed), to being slow on a lot of Android phones, and now something even more worrisome in terms of privacy, some users have had their messages sent to wrong recipients.
Apparently, this bug is affecting Gtalk users who are trying to talk to Hangouts users. Gtalk and Hangouts basically two different apps talking to each other, which means the potential for something breaking is a lot bigger than if a Gtalk user talked to another Gtalk user, or if one Hangouts user talked to another Hangouts user.
In a way, this is something to be expected as Google is trying to kill off Gtalk, and not maintain it fully anymore. Still, they should've been a lot more careful, as this sort of issue could cause a lot of embarrassment for some people if their chats end up sent to someone else. Google has already been in a lot of trouble with the whole PRISM scandal, and other privacy related scandals in which they were involved, so they need to be more careful now more than ever about how well they treat their users' privacy, whether on purpose or by mistake.
Google has promised to look into the issue:
"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."
If you want to avoid this issue until they fix it, then either stop using either Gtalk or Hangouts for now, or at least use only Hangouts, as it should be the more stable platform, and try to talk only with other Hangouts users, or tell your Gtalk friends to upgrade to Hangouts, too. Hangouts is not the ideal chat platform we were hoping it would be before launch, but it remains the main way to communicate with other Android and Gmail users.
Hopefully before too long, Google will also integrate it with SMS chat, to make it more like iMessage, and if they could also buy another large mobile chat platform, like Whatsapp, that could also make Hangouts a lot more ubiquitous, even for users of other platform. The more ubiquitous a chat platform is, there more likely it is all of your friends are using it, too, so that's definitely something Google shouldn't ignore.