It looks like single test procedure (testBranches @ MercurialSCMTest.java:47) is failing three different tests.

http://ci.hudson-labs.org/job/hudson_all_plugins/org.jvnet.hudson.plugins$mercurial/140/testReport/hudson.plugins.mercurial/CachingSCMTest/testBranches/
http://ci.hudson-labs.org/job/hudson_all_plugins/org.jvnet.hudson.plugins$mercurial/140/testReport/hudson.plugins.mercurial/DebugFlagTest/testBranches/
http://ci.hudson-labs.org/job/hudson_all_plugins/org.jvnet.hudson.plugins$mercurial/140/testReport/hudson.plugins.mercurial/MercurialSCMTest/testBranches/

Error Message null

Stacktrace:

junit.framework.AssertionFailedError: null
at junit.framework.Assert.fail(Assert.java:47)
at junit.framework.Assert.assertTrue(Assert.java:20)
at junit.framework.Assert.assertTrue(Assert.java:27)
at hudson.plugins.mercurial.MercurialTestCase.buildAndCheck(MercurialTestCase.java:47)
at hudson.plugins.mercurial.MercurialSCMTest.testBranches(MercurialSCMTest.java:47)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)