Update: WordPress 4.2.4 is out.
WordPress 4.2.4 Release Candidate 1 is now available and fixes issues with shortcodes and inline scripts, that were introduced with WordPress 4.2.3.
The WordPress 4.2.3 security release introduced problems with shortcodes in angle brackets <[shortcode]> and issues with inline scripts that used the CDATA block. Issues that caused scripts to be commented out and no longer run.
Example script with CDATA block:
<script>// <![CDATA[ _my_function('data'); // ]]> </script>
If you want to test WordPress 4.2.4 before the official release, you can download WordPress Release Candidate 1 here.