Got this from one run of one of our matrix builds:

FATAL: null
java.lang.NullPointerException
at hudson.matrix.DefaultMatrixExecutionStrategyImpl.waitForCompletion(DefaultMatrixExecutionStrategyImpl.java:241)
at hudson.matrix.DefaultMatrixExecutionStrategyImpl.run(DefaultMatrixExecutionStrategyImpl.java:163)
at hudson.matrix.MatrixBuild$MatrixBuildExecution.doRun(MatrixBuild.java:340)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:500)
at hudson.model.Run.execute(Run.java:1502)
at hudson.matrix.MatrixBuild.run(MatrixBuild.java:289)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:237)
at hudson.model.OneOffExecutor.run(OneOffExecutor.java:66)