Texas Water Code - WATER § 63.255. Validity of Bonds

(a) After the bonds are certified by the attorney general and registered by the comptroller, they shall be held prima facie valid and binding obligations in every action, suit, or proceeding in which their validity is brought into question.

(b) In any action brought to enforce collection of the bonds or interest on the bonds, the certificate of the attorney general or a certified copy of the certificate shall be received as prima facie evidence of the validity of the bonds and their coupons, and the only defense that can be offered against the validity of the bonds or coupons is forgery or fraud.