If a nickname has an underscore ( '_' ) in it, then gajim will bind alt+'next char' to its info.
If you have a contact that has an underscore ('_') in its nick-name or contact information ( i.e. blah_blah%blah.com@jabber.blah.com ) then gajim will bind the alt+'next char' ( i.e alt+b ) to displaying its information, witch can be really anoying if you're using alt+[1-9] to switch chat windows.