It appears that when a channel is "registered" on freenode, the server sends a reply of the following form to set the channel's URL:
:zelazny.freenode.net 328 rme #ccl :http://ccl.clozure.com/
CL-IRC doesn't know about this, and signals an error when it encounters it.
Anyway, it might be reasonable to add
(328 :rpl_channel_url)
to *REPLY-NAMES* in variables.lisp.
This event is mentioned in a few places I found via Google:
http://colloquy.info/project/browser/trunk/Chat%20Core/MVIRCNumerics.h http://www.mishscript.de/reference/rawhelp3.htm#raw328 http://www.alien.net.au/irc/irc2numerics.def