When trying to checkout a Mercurial repository, QuickBuilld threw an exception indicating an "unknown revision". I have only seen this once, as the previous and all subsequent runs performed without an issue. The output is given below:

15:05:24,162 INFO - Executing pre-execute action...
15:05:24,163 INFO - Running step...
15:05:24,171 INFO - Checking step execute condition...
15:05:24,173 INFO - Step execute condition satisfied, executing...
15:05:25,179 INFO - Executing pre-execute action...
15:05:25,188 INFO - Checking out Verification_tools repo at tip
15:05:25,188 INFO - Running step...
15:05:25,197 INFO - Getting changes of 'Verification_tools' since build '1.0.105'...
15:05:25,620 INFO - Getting changes between c311191f8c0dbd2ca34daf3429471c41e6966fed - b358dfb13eae3f59dc23f7fbb3ce234e6b722406
15:05:25,659 ERROR - abort: unknown revision 'c311191f8c0dbd2ca34daf3429471c41e6966fed'!
15:05:25,872 INFO - Executing post-execute action...
15:05:25,872 ERROR - Step 'master>ROOT-VerificationTools Checkout' is failed.
    com.pmease.quickbuild.QuickbuildException: Failed to run command: hg -y log -l1 --template={date|rfc3339date} -r c311191f8c0dbd2ca34daf3429471c41e6966fed
    Command return code: 255
    Command error output: abort: unknown revision 'c311191f8c0dbd2ca34daf3429471c41e6966fed'!
        at com.pmease.quickbuild.execution.Commandline$ExecuteResult.buildException(Commandline.java:442)
        at com.pmease.quickbuild.execution.Commandline$ExecuteResult.checkReturnCode(Commandline.java:457)
        at com.pmease.quickbuild.plugin.scm.helper.ScmCli.run(ScmCli.java:90)
        at com.pmease.quickbuild.plugin.scm.helper.ScmCli.run(ScmCli.java:79)
        at com.pmease.quickbuild.plugin.scm.helper.ScmCli.run(ScmCli.java:71)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgCli.getDateOfRevision(HgCli.java:218)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgCli.changesBetween(HgCli.java:230)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository.getChangesBetween(HgRepository.java:83)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.CGLIB$getChangesBetween$9(<generated>)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3$$FastClassByCGLIB$$3742a8b9.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:269)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.getChangesBetween(<generated>)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository.getChangesBetween(HgRepository.java:36)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.CGLIB$getChangesBetween$10(<generated>)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3$$FastClassByCGLIB$$3742a8b9.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:269)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.getChangesBetween(<generated>)
        at com.pmease.quickbuild.repositorysupport.Repository.getChangesSince(Repository.java:459)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.CGLIB$getChangesSince$74(<generated>)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3$$FastClassByCGLIB$$3742a8b9.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:269)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.getChangesSince(<generated>)
        at com.pmease.quickbuild.repositorysupport.Repository.getChanges(Repository.java:386)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.CGLIB$getChanges$56(<generated>)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3$$FastClassByCGLIB$$3742a8b9.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:269)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.getChanges(<generated>)
        at com.pmease.quickbuild.repositorysupport.Repository.getChanges(Repository.java:365)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.CGLIB$getChanges$55(<generated>)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3$$FastClassByCGLIB$$3742a8b9.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:269)
        at com.pmease.quickbuild.plugin.scm.mercurial.HgRepository$$EnhancerByCGLIB$$ba46cbb3.getChanges(<generated>)
        at com.pmease.quickbuild.plugin.basis.checkout.CheckoutStep.run(CheckoutStep.java:74)
        at com.pmease.quickbuild.plugin.basis.checkout.CheckoutStep$$EnhancerByCGLIB$$8a60c80b.CGLIB$run$2(<generated>)
        at com.pmease.quickbuild.plugin.basis.checkout.CheckoutStep$$EnhancerByCGLIB$$8a60c80b$$FastClassByCGLIB$$6cfa34ca.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:269)
        at com.pmease.quickbuild.plugin.basis.checkout.CheckoutStep$$EnhancerByCGLIB$$8a60c80b.run(<generated>)
        at com.pmease.quickbuild.stepsupport.Step.execute(Step.java:501)
        at com.pmease.quickbuild.stepsupport.StepExecutionJob.executeStepAwareJob(StepExecutionJob.java:29)
        at com.pmease.quickbuild.stepsupport.StepAwareJob.executeBuildAwareJob(StepAwareJob.java:47)
        at com.pmease.quickbuild.BuildAwareJob.execute(BuildAwareJob.java:61)
        at com.pmease.quickbuild.grid.GridJob.run(GridJob.java:78)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
        at java.util.concurrent.FutureTask.run(FutureTask.java:166)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
        at java.lang.Thread.run(Thread.java:722)