I have a field type in which the schema can be edited by the user in the UI. They can add or remove columns to the field schema, which means there also may be indexes that existed in my hook_field_schema() at one point when the field is created, but later does not exist. The problem I'm experiencing is that when field_update_field() is called which should drop the old field table and re-create with the current schema, it loads the 'old' indexes from the field_read_field() stored in the database. This causes a database exception because field_sql_storage attempts to create an index on a table column that does not exist.
It seems that field_create/update_field() supports passing in additional indexes that are not defined in hook_field_schema(), which frankly seems like a very bad idea.
This will need to be verified if this also is present on Drupal 8.