URLTrigger will not trigger when the cron-like 'spec' element is empty.
However, on saving the Jenkins job, it will delete the contents of this tag, so triggering never happens.

Working config:
---------------
<org.jenkinsci.plugins.urltrigger.URLTrigger>
<spec>* * * * *</spec>
<entries>
<org.jenkinsci.plugins.urltrigger.URLTriggerEntry>
<url>http://jenkins-dev.fortes.local:8080/job/Build%205-5-4/lastSuccessfulBuild/buildNumber</url>
<proxyActivated>false</proxyActivated>
<checkStatus>false</checkStatus>
<statusCode>200</statusCode>
<checkETag>false</checkETag>
<checkLastModificationDate>true</checkLastModificationDate>
<inspectingContent>true</inspectingContent>
<contentTypes>
<org.jenkinsci.plugins.urltrigger.content.SimpleContentType/>
</contentTypes>
</org.jenkinsci.plugins.urltrigger.URLTriggerEntry>
</entries>
<labelRestriction>false</labelRestriction>
</org.jenkinsci.plugins.urltrigger.URLTrigger>

Non-working (after modifying the job <spec> is cleared):
--------------------------------------

<org.jenkinsci.plugins.urltrigger.URLTrigger>
<spec></spec>
<entries>
<org.jenkinsci.plugins.urltrigger.URLTriggerEntry>
<url>http://jenkins-dev.fortes.local:8080/job/Build%205-5-4/lastSuccessfulBuild/buildNumber</url>
<proxyActivated>false</proxyActivated>
<checkStatus>false</checkStatus>
<statusCode>200</statusCode>
<checkETag>false</checkETag>
<checkLastModificationDate>true</checkLastModificationDate>
<inspectingContent>true</inspectingContent>
<contentTypes>
<org.jenkinsci.plugins.urltrigger.content.SimpleContentType/>
</contentTypes>
</org.jenkinsci.plugins.urltrigger.URLTriggerEntry>
</entries>
<labelRestriction>false</labelRestriction>
</org.jenkinsci.plugins.urltrigger.URLTrigger>

Jenkins 1.474 on Ubuntu 12.04
URLTrigger 0.23