Seen in Logstash for various page views of File-namespace pages with a custom uselang query parameter. It seems this is bypassing the makeKey() logic for that backend.
Exception from line 156 of /srv/mediawiki/php-1.30.0-wmf.4/includes/libs/objectcache/MemcachedBagOStuff.php: Key contains invalid characters: commonswiki:pcache:idhash:19264588-0!userlang=⧼lang⧽