Logback has been failing for quite some time on our build server.

copySLF4J_JAR:
[echo] Copying /Users/jenkins/.m2/repository/org/slf4j/slf4j-api/1.7.7/slf4j-api-1.7.7.jar to lib/
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Logback-Parent .................................... SUCCESS [ 0.804 s]
[INFO] Logback Core Module ............................... SUCCESS [ 22.700 s]
[INFO] Logback Classic Module ............................ FAILURE [ 25.905 s]
[INFO] Logback Access Module ............................. SKIPPED
[INFO] Logback Site ...................................... SKIPPED
[INFO] Logback Examples Module ........................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 54.240 s
[INFO] Finished at: 2014-05-16T18:35:28+01:00
[INFO] Final Memory: 39M/581M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-antrun-plugin:1.7:run (ant-osgi-test) on project logback-classic: An Ant BuildException has occured: The following error occurred while executing this line:
[ERROR] <[jenkins-server]/job/Logback/ws/logback-classic/osgi-build.xml>:46: Directory does not exist: <[jenkins-server]/job/Logback/ws/logback-examples/lib>
[ERROR] around Ant part ...<ant antfile="<[jenkins-server]/job/Logback/ws/logback-classic/osgi-build.xml"/>...> @ 7:89 in <[jenkins-server]/job/Logback/ws/logback-classic/target/antrun/build-main.xml>

This is likely caused by the fact that empty directories can't be added to a git repository.

I'd suggest to create the logback-examples/lib directory and add some dummy file to it. This would ensure that it's available after a clean checkout.