Example 1: MassMessage was recently fixed for dty.wikipedia (cf. T188014) however there's no CentralAuth data for MediaWiki message delivery for dty.wikipedia. MassMessage was recently used as well to send a message to wikimania2018.wikimedia.org. While the sending worked, there's no CentralAuth data for wikimania2018 either.
This is an issue as those accounts are becoming 'ghosts' and, for example, since they don't appear on CentralAuth, features such as GlobalUserPage won't work on them until they appear there.
Citing dtywiki, wikimania2018 and bdwikimedia as the two recent examples found out there. I don't know if there are more.