Post changes a username retroactively

My blog is configured in such a way that I am the admin, while new registered users are authors.

It happened to me twice that someone registered without posting anything, and later some of my own older posts “changed” to appear as if this someone had posted them.

Is it a bug? Some kind of a known/unknown hack? Perhaps some bug in the theme code displaying the posting user? Anything I can do about it?

Thanks.