When one child of a matrix job dies, all other children are killed. There appears to be a race condition between this and when email-ext is able to send email, because when this happens I sometimes don't get email. I have a degenerate case here where I have a matrix job with exactly 1 child. When the build completed it was unstable. However the child wasn't able to send email because it was killed too soon.

Output from master job:
20:31:36 Triggering mac
20:45:51 mac completed with result UNSTABLE
20:45:51 Interrupting #43
20:45:52 Finished: UNSTABLE

Output from slave:
20:45:43 [DRY] Collecting duplicate code analysis files...
20:45:44 [WARNINGS] Parsing warnings in console log with parsers [GNU compiler 4 (gcc)]
20:45:46 [WARNINGS] GNU compiler 4 (gcc) : Found 67 warnings.
20:45:47 [TASKS] Scanning workspace files for tasks...
20:45:47 [TASKS] Found 3 open tasks.
20:45:48 Archiving artifacts
20:45:49 Recording test results
20:45:51 Build step 'Publish JUnit test result report' changed build result to UNSTABLE
20:45:51 Email was triggered for: Unstable
20:45:51 Sending email for trigger: Unstable
20:45:52 ERROR: Could not send email as a part of the post-build publishers.
20:45:52 java.lang.InterruptedException
20:45:52 at java.lang.Object.wait(Native Method)
20:45:52 at hudson.remoting.Request.call(Request.java:127)
20:45:52 at hudson.remoting.Channel.call(Channel.java:681)
20:45:52 at hudson.EnvVars.getRemote(EnvVars.java:202)
20:45:52 at hudson.model.Computer.getEnvironment(Computer.java:821)
20:45:52 at hudson.model.Run.getEnvironment(Run.java:1825)
20:45:52 at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:809)
20:45:52 at hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:295)
20:45:52 at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:235)
20:45:52 at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:227)
20:45:52 at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:189)
20:45:52 at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
20:45:52 at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:694)
20:45:52 at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:669)
20:45:52 at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:647)
20:45:52 at hudson.model.Build$RunnerImpl.cleanUp(Build.java:171)
20:45:52 at hudson.model.Run.run(Run.java:1448)
20:45:52 at hudson.matrix.MatrixRun.run(MatrixRun.java:146)
20:45:52 at hudson.model.ResourceController.execute(ResourceController.java:88)
20:45:52 at hudson.model.Executor.run(Executor.java:230)
20:45:52 Finished: UNSTABLE