This is just a short description of an error that I have encountered in Sitecore Publishing Server v. 2.1.0. If you have an item with two versions and the second version has a publish from date set but is also marked as unpublishable then the previous version will not be published.
I have reported the error to Sitecore support and they have provided me with a patch https://sitecore.app.box.com/s/i6um06qwtjyntzl6kp408dpgf7fv9xdl.