breaks submit handlers of other forms

    // Replace submit callback and use our own function.
    $form['#submit'] = str_replace('system_settings_form_submit', 'variable_settings_form_submit', $form['#submit']);

This breaks the form submit handlers of forms that have a submit handler whose name contains 'system_settings_form_submit'. For example, https://www.drupal.org/project/taxonomy_edge has a taxonomy_edge_system_settings_form_submit_custom(). Variable module rewrites this to a function that doesn't exist, causing the Taxonomy Edge configuration form to crash on submit.