Very occasionally – maybe once every 4-5 months, on a job that's run about a dozen times per day – I'll get a "no such property" error for a property that's defined in a loaded file.

Our code is something like this:

alertMsgs = load("${pwd()}@script/jobs/constants.groovy");
echo(alertMsgs.SUCCESS);

and jobs/constants.groovy looks like this:

SUCCESS = "...";
return this;

Normally this works exactly as expected, but every so often we get this stacktrace:

groovy.lang.MissingPropertyException: No such property: SUCCESS for class: groovy.lang.Binding
at groovy.lang.Binding.getVariable(Binding.java:63)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onGetProperty(SandboxInterceptor.java:224)
at org.kohsuke.groovy.sandbox.impl.Checker$4.call(Checker.java:241)
at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:238)
at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.getProperty(SandboxInvoker.java:28)
at com.cloudbees.groovy.cps.impl.PropertyAccessBlock.rawGet(PropertyAccessBlock.java:20)
at WorkflowScript.finishWithSuccess(WorkflowScript:732)
at onMaster.call(/mnt/jenkins_jobs/deploy/jobs/deploy-webapp/builds/882/libs/kautils/vars/onMaster.groovy:12)
at withTimeout.call(/mnt/jenkins_jobs/deploy/jobs/deploy-webapp/builds/882/libs/kautils/vars/withTimeout.groovy:60)

 
What could be causing this?