Go to nnvirtual group, read one article and then type ^.
You'll get:
Variable binding depth exceeds max-specpdl-size
I confirmed with latest Gnus 5.8.4 from CVS.

Because gnus-override-method is bound,
gnus-find-method-for-group always returns nnvirtual.  Thus,
it ends up in an infinite loop.
I don't know this is the right fix, but at least it works
for me.  Latter half is to prevent an error,
Wrong type argument stringp: nil

2000-02-21  Yoshiki Hayashi  <yoshiki@xemacs.org>

	* nnvirtual.el (nnvirtual-request-article):
	Bind gnus-override-method to nil.
	(nnvirtual-request-update-mark): Don't update mark when
	article is not there.