>>>>> "Paul" == Paul Stodghill <stodghil@cs.cornell.edu> writes:

Paul> I started using the agent this morning, and I have encountered
Paul> the following error intermittently throughout the day.

Paul> Signaling: (error "Selecting deleted or non-existent buffer")
Paul>   copy-to-buffer(#<killed buffer> 1 233)
Paul>   gnus-agent-fetch-headers("comp.emacs")
Paul>   gnus-agent-fetch-group-1("comp.emacs" (nntp "newsstand.cit.cornell.edu"))
Paul>   gnus-agent-fetch-session()
Paul>   (progn (gnus-agent-fetch-session))
Paul> )
Paul>   (if gnus-plugged (progn (gnus-agent-fetch-session)))
Paul> )
Paul>   (when gnus-plugged (gnus-agent-fetch-session))
Paul> )
Paul>   pvs-fetch-session-after-new-news()
Paul>   run-hooks(pvs-fetch-session-after-new-news)
Paul>   apply(run-hooks pvs-fetch-session-after-new-news)
Paul>   gnus-run-hooks(gnus-after-getting-new-news-hook)
Paul>   gnus-group-get-new-news(nil)
Paul>   call-interactively(gnus-group-get-new-news)

Paul> I am not sure what is triggering it, but once I encounter it, I
Paul> have to quit and restart Gnus to make it go away.

I had encountered this before (using batch-fetch). When I set a trap,
it never happened again. OK, now I get your backtrace.

You probably use more than one nntp server.  I guess you lose
connection to one nntp server (probably not newsstand.cit.cornell.edu)

If my guess is right, the reason is that gnus-agent uses
gnus-agent-overview-buffer as nntp-server-buffer.  When gnus can not
connect a nntp server, the buffer is killed.

,-------- Line 959, file gnus-agent.el
|       (let ((nntp-server-buffer gnus-agent-overview-buffer))
| 	(setq gnus-newsgroup-headers
| 	      (gnus-get-newsgroup-headers-xover articles nil nil group)))
`--------

Anyway, try this patch.

1999-07-12  Shenghuo ZHU  <zsh@cs.rochester.edu>

	* gnus-agent.el (gnus-agent-fetch-group-1): Recreate agent
 	overview buffer if it is killed.