Bad coding in a development partner's custom module was the culprit. It was overriding content_profile_registration's setting of a validation function (and every other module's, too).

Inside the custom module's implementation of hook_form_alter() lives the error:
<?php
switch ($form_id) {
// ... ignore lots of code
case 'user_register':
// ... ignore more code

Drag and drop into the TinyMCE textarea triggers onEvent twice in such close succession that it creates a race condition, and the same values for the length of the textarea before and after are given.

Possible culprits:

./profiles/scf/modules/modified/user_import/user_import.module:1450: user_delete($existing, $existing->uid);
./profiles/scf/modules/modified/invite/invite_cancel_account.module:20: user_delete(array(), $account->uid);
./profiles/scf/modules/contrib/logintoboggan/logintoboggan.module:42: user_delete(array(), $account->uid);

Earl's long quest brought this:

This could be prettier. Do not judge. Especially could be prettier using an IDE.

<?php
/**
* Invoke a hook_nodeapi() operation in all modules.
*
* @param &$node
* A node object.
* @param $op
* A string containing the name of the nodeapi operation.
* @param $a3, $a4
* Arguments to pass on to the hook, after the $node and $op arguments.
* @return

This code (which is not called automatically but is a helper function for our hook_form_alter implementation) adds the suffix but does not change the forms size. Somehow these widget settings must get added later.

<?php
function biblioreference_alter_item(&$form, $key, $item, $fields) {
$field_name = strstr($key, 'field_');