Certain versions of Tomcat with certain configurations can leave log4j in a corrupted state that is unreachable by normal language means as the Tomcat classloader will set to null private static members in order to prevent a class of memory leaks. It would be fine if that only occurred after the class was no longer used, but in some poorly defined instances, log4j will still be referenced after the Tomcat classloader plays with it. The error indicates that a log request was attempted after Tomcat played with it. Prior to log4j 1.2.15, that would result in a NullPointerException. In log4j 1.2.15 and later, it results in this error message and all subsequent logging requests are discarded. If this happens only on shutdown, you may not have missed anything.

For more background than you probably want, see the Tomcat and log4j bugs:

http://issues.apache.org/bugzilla/show_bug.cgi?id=41939
http://issues.apache.org/bugzilla/show_bug.cgi?id=40212
http://issues.apache.org/bugzilla/show_bug.cgi?id=43867