Odd little problem on my site today.
I had a new user register for the site. In his introduction post, he stated that he noticed several other posts in the forum as being posted under his user name.
I run a small forum, so I know all the user names, and this was the first time this user or name or e-mail had been used.
I tracked the other posts that he was talking about. I did find a few old posts that had been posted by another user that were now changed to show as being posted by this user, like it took over the other users posts for a few threads.
The old threads were from May. This new user just registered today, but the posts from May now show as being posted by him.