Problem/Motivation
If someone tries to insert a long term via autocomplete on a page, then it will result in an exception because no validation is fired that the term isn't too long.
This is also an issue on D7, which is where I originally uncovered the situation. I had expected it to be fixed in D8 with typed data, but that isn't the case.
Drupal\Core\Entity\EntityStorageException: SQLSTATE[22001]: String data, right truncated: 1406 Data too long for column 'name' at row 1: INSERT INTO {taxonomy_term_field_data} (tid, vid, langcode, name, description__value, description__format, weight, changed, default_langcode) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8); Array ( [:db_insert_placeholder_0] => 2 [:db_insert_placeholder_1] => tags [:db_insert_placeholder_2] => en [:db_insert_placeholder_3] => Donec mi odio; faucibus at; scelerisque quis; convallis in; nisi; Proin sapien ipsum;porta a; auctor quis; euismod ut; mi; Sed libero; Praesent vestibulum dapibus nibh; Pellentesque auctor neque nec urna; Aliquam lorem ante; dapibus in; viverra quis; feugiat a; tellus; Vivamus aliquet elit ac nisl; Donec vitae orci sed dolor rutrum auctor; Donec pede justo; fringilla vel; aliquet nec; vulputate eget; arcu; Donec venenatis vulputate lorem [:db_insert_placeholder_4] => [:db_insert_placeholder_5] => [:db_insert_placeholder_6] => 0 [:db_insert_placeholder_7] => 1444326833 [:db_insert_placeholder_8] => 1 ) in Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 757 of /home/dfv2i/www/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php).
Proposed resolution
Remaining tasks
Fix it. I think a good starting point would be EntityAutocomplete->validateEntityAutocomplete().