> However, I don't have any good idea, so I decided to ask here.  does
> anyone has any idea?
> here is one liner to produce the error message.
> $ ruby -e "Thread.current.join"
> -e:1:in `join': recursive join (ThreadError)
>         from -e:1

I'd suggest "circular join", just like "circular reference", "circular
inheritance", "circular dependency", ... if I understand what it is about. 

matju