Earlier this week I wasted a couple of hours of my life dealing with this unforeseen issue. Usually the log4j team (to which I used to belong) is very very good about considering various deployment scenarios and environments, and taking the safe route. So when I updated from 1.2.14 to 1.2.15 I expected no issues. I did read the changelog, which mentioned the move to a Maven build, but that did not alarm me as I like Maven (2, not 1 quite as much) and use it anyways.
Instead, I got into a Maven / Java equivalent of DLL Hell. log4j 1.2.15 has new compile-scope dependencies on a bunch of stuff, including the JMX (Java Management Extensions) libraries. This sucks if you're running on J2SE 5.0 or later, as I do, because those JREs already include the JMX libraries.
The exact symptoms vary according to your application or server environment, but in my case Tomcat and Jetty both complained about different versions of the MBeanServer on the classpath.
It took some digging, and I found some helpful blog posts. After mucking around with dependency exclusion and trying to change the scope of dependencies, I just decided to go back to log4j 1.2.14, which works well.