With a simple web-app that does not automatically create a session, using the following pattern in logback-access.xml:
{code:xml}
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%h %l %u %user %date "%r" %s %b %sessionID</pattern>
    </encoder>
  </appender>
{code}

will cause the following exception:

{code}
20:22:26,782 |-ERROR in ch.qos.logback.core.ConsoleAppender[STDOUT] - Appender [STDOUT] failed to append. java.lang.IllegalStateException: Cannot create a session after the response has been committed
    at java.lang.IllegalStateException: Cannot create a session after the response has been committed
    at     at org.apache.catalina.connector.Request.doGetSession(Request.java:3030)
    at     at org.apache.catalina.connector.Request.getSession(Request.java:2451)
    at     at ch.qos.logback.access.spi.AccessEvent.getSessionID(AccessEvent.java:249)
    at     at ch.qos.logback.access.pattern.SessionIDConverter.convert(SessionIDConverter.java:21)
...
{code}

I believe that you can resolve this by changing AccessEvent.getSession() to call getSession(false) rather than getSession().