Ala. Code § 16-13-306

Warrants reciting that they are issued pursuant to the terms of this article shall in any action or proceeding involving their validity be conclusively deemed to be fully authorized thereby and to have been issued, sold, executed, and delivered in conformity therewith and with all other provisions of law applicable thereto and shall be incontestable, anything herein or in other statutes to the contrary notwithstanding.