This is related to JENKINS-2180.

I'm using the URL Copy SCM, polling every 5 minutes for changes. I wanted to trigger the build on updates to the url(s), but not until they've settled down. To this end, I use the quiet period functionality. I set the quiet period to 315 seconds (15 seconds more than my polling interval). However, since enabling this, no builds have happened at all. This seems to be because when it polls, it compares to the workspace rather than the results of the preceding polling.