Currently, the logback-acces documentation (located at http://logback.qos.ch/access.html#configuration ) states twice that the "combined" format has the following format:
%h %l %u %t "%r" %s %b "%i

{Referer}" "%i{User-Agent}"

This is wrong, both from what logback uses as the real format when using "<pattern>combined</pattern>" and from what the format is "defined" to be (as defined by Apache?).

At both places http://logback.qos.ch/access.html#configuration should instead state
%h %l %u [%t] "%r" %s %b "%i{Referer}

" "%i

{User-Agent}

"
(notice the brackets around "%t").

Compare to
http://logback.qos.ch/manual/layouts.html#AccessPatternLayout
http://httpd.apache.org/docs/1.3/logs.html#combined