OK, I figured it out. I'm using the znc IRC bouncer (the version in Hardy), and 73_freenode_ircd-seven.dpatch in Lucid's xchat causes xchat to send 'CAP LS', which makes znc unhappy. I also can't find reference to it in the IRC RFCs, so I'm not sure what the server is supposed to do in this case.
OK, I figured it out. I'm using the znc IRC bouncer (the version in Hardy), and 73_freenode_ ircd-seven. dpatch in Lucid's xchat causes xchat to send 'CAP LS', which makes znc unhappy. I also can't find reference to it in the IRC RFCs, so I'm not sure what the server is supposed to do in this case.