The other day, I was going through system mail to make sure everything was working (and reporting) properly. A script that I had put in /etc/cron.daily wasn't sending me any mail, which was odd: it ran fine when I started it manually. After a little digging, it turns out that run-parts will ignore some items based on namespace requirements and my script had a .sh suffix. I know it's not a necessary thing to have, but it helps me keep track at a glance what stuff I've put in that directory. So, running run-parts --test /your/directory will tell you what run-parts will actually run. If you want to know more about the namespace requirements, check out the man page for run-parts.