The DB field for forum_posts->subject is varchar(255), but the web form does not enforce this. If a longer subject is entered, a 'Could not add your post due to an unknown error' is thrown when on Postgres, under MySQL the subject just gets truncated by the DB, with no knowledge of the user.

Solution is to add this at line 118 of mod/forum/post_form.php:
$mform->addRule('subject', get_string('maximumchars', '', 255), 'maxlength', 255, 'client');

Same problem found with the forum name. Should add this at like 17 of mod/forum/mod_forum.php:
$mform->addRule('name', get_string('maximumchars', '', 255), 'maxlength', 255, 'client');