Some of my users get a security warning when they view pages with the +1 button. How do I get rid of this?

The +1 button code requires a script from Google's servers. You can include this script using either http:// or https://, like this:

<script src="https://apis.google.com/js/plusone.js"></script>

If your web page uses https://, some browsers and verification tools will show an error when any assets on the page are called via http://. If your site serves pages via https://, make sure that the +1 button code on those pages also uses https://. (In fact, it's fine to use https:// in the button code for all pages, even if they are only served via http://.)