If proxy scm is set to point to a project that no longer exists (i.e. rename or delete) - the job execution just fails with:

{{2015-09-14 05:25:09.820 | FATAL: null
2015-09-14 05:25:09.820 | java.lang.NullPointerException
2015-09-14 05:25:09.820 | at hudson.plugins.templateproject.ProxySCM.checkout(ProxySCM.java:91)
2015-09-14 05:25:09.821 | at hudson.scm.SCM.checkout(SCM.java:485)
2015-09-14 05:25:09.821 | at hudson.model.AbstractProject.checkout(AbstractProject.java:1282)
2015-09-14 05:25:09.821 | at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:610)
2015-09-14 05:25:09.821 | at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
2015-09-14 05:25:09.821 | at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:532)
2015-09-14 05:25:09.821 | at hudson.model.Run.execute(Run.java:1741)
2015-09-14 05:25:09.821 | at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
2015-09-14 05:25:09.821 | at hudson.model.ResourceController.execute(ResourceController.java:98)
2015-09-14 05:25:09.821 | at hudson.model.Executor.run(Executor.java:381)}}

Would be better to specifically check for that scenario and output a useful error message.