Sometimes, the server thinks you are another user. If you were viewing a page, the server generates the page based on their settings. It is most obvious when you look in the title and you see someone elses username. More often than not, refreshing gets you yourself back.
Suppose someone is updating their profile information, and at that moment, the server thinks they are someone else. It could, in theory, change the wrong user's profile. I've had my email disappear and theme change, and changing them back fixes things for "a good while".
It has happened that a line in chat appeared with the wrong username...
Posts with the wrong author, I haven't seen yet...
I feel bad reporting so many bugs, cause Vatsis is already so busy working on other stuff