A very strange thing happened to a member who posted about her experience in Google Groups. Apparently, while logged into her Google account, she was browsing forums threads. When trying to respond to messages, she noticed that replying failed. It was at that point that she noticed that she was logged in as a different user. The strange thing is that she's never seen this email address before. Is Google accidentally mixing up user login sessions? It has happened to the particular user with multiple unknown email addresses.
Susan Moskwa updated the thread to report that Google treated this as a high priority issue. Adam Lasnik later reported that Google has completely resolved the problem. The affected user later confirmed that she has not run into this issue any longer.
Forum discussion at Google Groups.