There is a serious problem going on currently with messages sent from some users via Google Hangouts. The problem is that some senders are seeing their chat messages sent to the wrong recipients. The problem has reportedly been occurring since about 4 PM EST.
The users report that during chats with a specific person the messages are somehow being accidentally sent to other users of the chat service. Google has reportedly launched the issue and is currently looking into what might be causing the problem.
Currently there is no rhyme or reason to who the glitch affects and who it doesn't affect. There also is no clear pattern having the glitch does occur as to which recipients will receive the messages. So far, there's no indication that some sort of nefarious action is the cause of this flaw.
A Google spokesperson said:
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.
Hopefully we'll hear what's causing this issue soon. Until it's resolved the only thing you can do is not use these services or at least be very careful what you say.