Before updating to v1.37 (from 1.36) date_popup fields were correctly validated. In v1.37, no date is considered valid.
Consider a date_popup field that is created via the FAPI like this:
$form['date_donated'] = array( '#type' => 'date_popup', '#date_format' => 'm-d-Y', '#default_value' => date('m-d-Y H:i:s', REQUEST_TIME), '#required' => true, ),
No date is considered to be valid. I am using v7.x-2.6 of the Date module, of which the date_popup is a part.