Has a former President ever come out and said that a bill he signed into law is unconstitutional? Does he get a Mulligan?

Because President Bill Clinton did just that.

 As the president who signed the act into law, I have come to believe that DOMA is contrary to those principles and, in fact, incompatible with our Constitution.

While I agree with the substantive outcome of nixing DOMA, the precedents that this President is setting with respect to the scope of executive power is quite troubling for what a future President, whom I may disagree with, will do.