Sore error conditions are not detected and then puts the job in a certain state that causes other plugins that don't have good error checking to fail.
(This is related to JENKINS-30466)

e.g.
P4 Task: syncing files at change: 142720
... p4 sync c:\jenkins\workspace__atf_int_mlb_0/...@142720
+
duration: (519ms)

P4 Task: unshelve review: 142711
... p4 unshelve -f -s142711 -cdefault
+
... p4 revert -k c:\jenkins\workspace__atf_int_mlb_0/...
+
... duration: (35ms)
P4 Task: saving built changes.
... p4 client -o __atf_int_mlb_0_jenkins_p4_2_mlb-TEST-gb-mlb-dt-018 +
... p4 change -o 142711 +
Unable to get changes: com.perforce.p4java.exception.RequestException: Change 142711 unknown.

... p4 client -o __atf_int_mlb_0_jenkins_p4_2_mlb-TEST-gb-mlb-dt-018 +
... p4 change -o 142711 +
Unable to get changes: com.perforce.p4java.exception.RequestException: Change 142711 unknown.

... done

Maybe the user has deleted the shelved changes or even deleted the cl, or if the shelved cl is on a stream where it can't be handled.

Anyway, the plugin is not in a bad state, it doesn't inform the job so the normal builds steps go.

When the plugin is in this state the job fails at the end with the following:
FATAL: null
java.lang.NullPointerException
Collecting metadata...
Metadata collection done.
Finished: FAILURE

The Jenkins log show the following:
Sep 16, 2015 9:43:32 AM org.apache.commons.jelly.expression.jexl.JexlExpression evaluate
WARNING: Caught exception evaluating: cs.affectedFiles. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
at sun.reflect.GeneratedMethodAccessor688.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.commons.jexl.util.PropertyExecutor.execute(PropertyExecutor.java:125)
at org.apache.commons.jexl.util.introspection.UberspectImpl$VelGetterImpl.invoke(UberspectImpl.java:314)
at org.apache.commons.jexl.parser.ASTArrayAccess.evaluateExpr(ASTArrayAccess.java:185)
at org.apache.commons.jexl.parser.ASTIdentifier.execute(ASTIdentifier.java:75)
at org.apache.commons.jexl.parser.ASTReference.execute(ASTReference.java:83)
at org.apache.commons.jexl.parser.ASTReference.value(ASTReference.java:57)
at org.apache.commons.jexl.parser.ASTReferenceExpression.value(ASTReferenceExpression.java:51)
at org.apache.commons.jexl.ExpressionImpl.evaluate(ExpressionImpl.java:80)
at org.apache.commons.jelly.expression.jexl.JexlExpression.evaluate(JexlExpression.java:69)
at org.apache.commons.jelly.expression.jexl.JexlExpressionFactory$ExpressionSupportLocal.evaluate(JexlExpressionFactory.java:122)
at org.apache.commons.jelly.expression.ExpressionSupport.evaluateRecurse(ExpressionSupport.java:61)
at org.apache.commons.jelly.expression.ExpressionSupport.evaluateAsIterator(ExpressionSupport.java:94)
at org.apache.commons.jelly.tags.core.ForEachTag.doTag(ForEachTag.java:89)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161)
at org.apache.commons.jelly.tags.core.ForEachTag.doTag(ForEachTag.java:150)
at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161)
at org.apache.commons.jelly.impl.StaticTag.doTag(StaticTag.java:65)
at org.apache.commons.jelly.impl.StaticTagScript.run(StaticTagScript.java:124)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$1.run(CoreTagLibrary.java:98)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161)
at org.apache.commons.jelly.impl.StaticTag.doTag(StaticTag.java:65)
at org.apache.commons.jelly.impl.StaticTagScript.run(StaticTagScript.java:124)
at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95)
at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105)
at hudson.plugins.emailext.plugins.content.JellyScriptContent.convert(JellyScriptContent.java:158)
at hudson.plugins.emailext.plugins.content.JellyScriptContent.renderContent(JellyScriptContent.java:149)
at hudson.plugins.emailext.plugins.content.JellyScriptContent.evaluate(JellyScriptContent.java:58)
at org.jenkinsci.plugins.tokenmacro.DataBoundTokenMacro.evaluate(DataBoundTokenMacro.java:189)
at org.jenkinsci.plugins.tokenmacro.TokenMacro.expand(TokenMacro.java:182)
at org.jenkinsci.plugins.tokenmacro.TokenMacro.expandAll(TokenMacro.java:233)
at hudson.plugins.emailext.plugins.ContentBuilder.transformText(ContentBuilder.java:72)
at hudson.plugins.emailext.ExtendedEmailPublisher.getContent(ExtendedEmailPublisher.java:659)
at hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:538)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:306)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:297)
at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:244)
at org.jenkins_ci.plugins.run_condition.BuildStepRunner$2.run(BuildStepRunner.java:110)
at org.jenkins_ci.plugins.run_condition.BuildStepRunner$Fail.conditionalRun(BuildStepRunner.java:154)
at org.jenkins_ci.plugins.run_condition.BuildStepRunner.perform(BuildStepRunner.java:105)
at org.jenkins_ci.plugins.flexible_publish.ConditionalPublisher.perform(ConditionalPublisher.java:183)
at org.jenkins_ci.plugins.flexible_publish.FlexiblePublisher.perform(FlexiblePublisher.java:116)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:770)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:734)
at hudson.model.Build$BuildExecution.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:683)
at hudson.model.Run.execute(Run.java:1783)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
Caused by: java.lang.UnsupportedOperationException: getAffectedFiles() is not implemented by this SCM
at hudson.scm.ChangeLogSet$Entry.getAffectedFiles(ChangeLogSet.java:242)
... 59 more

I think the email plugin is trying to get the list of users and this is null (and it doesn't do very good checking?)

The Jenkins summary page shows:
Summary Of Changes - View Detail
No connection to Perforce.