Ex post facto

One of them added that it was hard lines that there wasn’t any gentleman “such-like as yourself, squire,” to show some sort of appreciation of their efforts in a liquid form; […] The opportunities of acquiring an abnormal thirst had been here limited; a noble use of them had, however, been made, and again I was compelled to deal with the result in an ex post facto manner.

Coupled with the Constitution's proscription of ex post facto laws is a similar prohibition against bills of attainder.

That the tax would take effect after some of the payments were made also raises issues. Says former Attorney General Richard Thornburg, also in an e-mail: "Such legislation could well run afoul of constitutional restrictions on bills of attainder, ex post facto laws, laws impairing the obligations of contract, unauthorized takings of property and the like."

What he did was not a crime when he did it, but he was convicted ex post facto.