The code in Executor does not check created tasks for null => NPE appears in logging procedures when calling executable.toString() in Executor::run(). This behavior causes hardly analyzable issues like JENKINS-18164

The issue sometimes appears for MatrixConfiguration runs, but according to the code it can be also reproduced in 1.574

Console output for runs:

   Jul 31, 2014 3:04:19 PM hudson.model.Executor run
   SEVERE: Executor threw an exception
   java.lang.NullPointerException
           at hudson.model.Executor.run(Executor.java:234)