As discovered when testing MDLSITE-5258, after upgrading to 3.4, tag pages which contain the Flickr block display a notice

Notice: block_tag_flickr::get_content(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "object" of the object you are trying to operate on was loaded before unserialize() gets called or provide a __autoload() function to load the class definition in /var/www/vhosts/moodle.org/html/blocks/tag_flickr/block_tag_flickr.php on line 84