Recently some backwards incompatible changes were made to the MobileWebSectionUsage schema. In addition, an array field was added without the items type specified.
Removing required fields is not allowed, and adding new required fields is not allowed. The only schema changes that are allowed are to add new optional fields.
I just edited https://wikitech.wikimedia.org/wiki/Analytics/Systems/EventLogging/Schema_Guidelines#Schema_set_up to indicate that array items are required. Otherwise we can't create Hive tables as we don't know the type of the array items, and get errors like:
RefineTargetException: Failed refinement of hdfs://analytics-hadoop/wmf/data/raw/eventlogging/eventlogging_MobileWebSectionUsage/hourly/2019/09/03/14 -> `event`.`MobileWebSectionUsage` (year=2019,month=9,day=3,hour=14). Original exception: java.lang.IllegalArgumentException: `initiallyOpenSections` array schema did not specify the items field)