While trying to use logbacks log4j bridge to have JBoss EmbeddedEJB3 to log I stumbled over the following stacktrace.
I use logback-0.9.7 and slf4j-1.4.0.

Caused by: java.lang.IllegalStateException: Unknown Priority TRACE
at org.apache.log4j.Category.priorityToLevel(Category.java:351)
at org.apache.log4j.Category.log(Category.java:332)
at org.jboss.logging.Log4jLoggerPlugin.trace(Log4jLoggerPlugin.java:119)
at org.jboss.logging.Logger.trace(Logger.java:152)
at org.jboss.kernel.plugins.bootstrap.AbstractBootstrap.run(AbstractBootstrap.java:93)
at org.jboss.ejb3.embedded.EJB3StandaloneBootstrap.createKernel(EJB3StandaloneBootstrap.java:422)
at org.jboss.ejb3.embedded.EJB3StandaloneBootstrap.boot(EJB3StandaloneBootstrap.java:375)