When the Object Cache of W3TC is enabled the plugin creates a wp-content/object-cache.php file.
If the Object Cache is later disabled, however, the object-cacche.php file persists.
Since the existence of object-cache.php affects $_wp_using_ext_object_cache WordPress will not behave the same after disabling the Object Cache as it did before it was enabled.
Should object-cache.php be removed if the Object Cache is no longer enabled?
What affect/purpose does the W3TC object-cache.php have if Object Cache is disabled?