When upgrading from an ancient MediaWiki version (1.12) the update.php script fails with the error:
An error occurred: A database query error has occurred. Did you forget to run your application's database schema updater after upgrading? Query: SELECT vt_tag AS `value` FROM `valid_tag` Function: PopulateChangeTagDef::setUserDefinedTags Error: 1146 Table 'wikidb.valid_tag' doesn't exist (localhost)
The valid_tag table was added in 1.15 and removed in 1.33: https://www.mediawiki.org/wiki/Manual:Valid_tag_table
Apparently, a script is running to migrate the data from that table to other tables before removing it, but that table doesn't exist in the first place. This means installations upgrading from a version before 1.15 will fail.
Reports of this error in the Support Desk: