Thread:Project:Support desk/Advice/instructions for creating a global login across several wikis/reply (7)

That makes life more difficult. Basically the shared user table has to be something that makes sense to both versions of MediaWiki. Using the same version of MediaWiki for all your wikis will reduce the chance of something not working. However, with that said, the main changes to user related tables introduced between MW 1.17 and MW 1.18: None of these are exactly going to cause major back compatibility issues that I can think of (Most commonly [but not neccesarily always] the newer db layout works with older MediaWiki). What you'd probably want to do is make sure the shared tables are updated to mediawiki 1.18 version of schema. Thus you'd want to run php update.php --doshared from your 1.18 install (but don't --doshared from your 1.17 install).
 * There's a new index on user_email
 * in the user_properties table, up_property now has max length of 255 bytes (up from 32)
 * And the user_former_groups table was added.