Create a simple form with a number field with a minimum number required and an unlimited option with a 0 value:
$form = new \IPS\Helpers\Form;
$form->add( new \IPS\Helpers\Form\Number( 'test', 0, FALSE, array( 'min' => 2, 'unlimited' => 0, 'unlimitedLang' => 'disabled' ) ) );
Now load the form, uncheck the Disabled checkbox, and submit the form. Once you've done all the steps this is the error that appears right under the number field:
The error makes you believe you have to insert at least 3 but the field does indeed accept a 2 value. The correct error should instead be: