Upgraded my multisite to W3TC 0.9.2.8 after the fiasco with 0.9.2.7 not working with cPanel or CloudFlare and 0.9.2.6 not working with BackupBuddy and got this error message in various parts of the admin area when performing different actions:
Fatal error: Call to undefined function send_origin_headers() in (WordPress root)/wp-admin/admin-ajax.php on line 20
Seems like this call comes up anytime the WP admin area uses AJAX to load part of the page. Some examples are the various widgets on the dashboard, approving/disapproving/trashing comments, and saving some settings.
Deactivating the W3 Total Cache plugin fixes this problem. Still don't know if the BackupBuddy problem is solved yet.