I just noticed something today.
I tend to edit comments in order to reply to them. That means I write the answer in the same field they did.
Now WordPress thinks I am that user (which is ok for me). But somehow they get "promoted" to Members during that event.
I noticed that because I have the wp-useronline plugin installed and as soon as I edit a comment like that I see myself with the name of the person as a member.
Why is that? I don't even have feature for others to register as members enabled.
Sounds more like a bug to me?