After yesterday's deployment, PHP started logging a warning on occasion:
May 15 19:55:15 10.64.16.70 apache2[9389]: PHP Warning: array_key_exists() expects parameter 2 to be array, null given in /usr/local/apache/common-local/php-1.22wmf3/extensions/ZeroRatedMobileAccess/includes/PageRenderingHooks.php on line 564
Version: unspecified
Severity: blocker