The following scenario leads to failing builds because of faulty behaviour of Jenkins:

• create a global environment variable:
• create a job (maven or freestyle; did not test it on matrix)
• configure maven: (enter the global env variable)
• run the job
• you get something like:

  Modules changed, recalculating dependency graph
  maven-agent.jar already up to date
  classworlds.jar already up to date
  maven-interceptor.jar already up to date
  maven2.1-interceptor.jar already up to date
  [amavenjob] $ java -Xmx512m -XX:MaxPermSize=256m -cp PATH/jenkins-slave/maven-agent.jar:PATH/jenkins-slave/classworlds.jar hudson.maven.agent.Main PATH/maven/current PATH/jenkins-slave/slave.jar PATH/jenkins-slave/maven-interceptor.jar 40919 PATH/jenkins-slave/maven2.1-interceptor.jar
  
  <===[JENKINS REMOTING CAPACITY]===>channel started
  
  log4j:WARN No appenders could be found for logger (org.apache.commons.beanutils.converters.BooleanConverter).
  log4j:WARN Please initialize the log4j system properly.
  
  Executing Maven:  -B -f PATH/jenkins-slave/workspace/amavenjob/pom.xml clean install ${JOB_MVN_PARAM}
  [INFO] Scanning for projects...
  [INFO] ------------------------------------------------------------------------
  [ERROR] BUILD FAILURE
  [INFO] ------------------------------------------------------------------------
  [INFO] Invalid task '${JOB_MVN_PARAM}': you must specify a valid lifecycle phase, or a goal in the format plugin:goal or pluginGroupId:pluginArtifactId:pluginVersion:goal
  [INFO] ------------------------------------------------------------------------
  [INFO] For more information, run Maven with the -e switch
  [INFO] ------------------------------------------------------------------------
  [INFO] Total time: < 1 second
  [INFO] Finished at: Fri Jan 31 09:12:40 CET 2014
  [INFO] Final Memory: 3M/59M
  [INFO] ------------------------------------------------------------------------
  
  channel stopped
  Finished: FAILURE