"The assessment of taxes collected erroneously is unjust enrichment. The
COMMON LAW LIABILITY of the COLLECTOR TO REFUND THE MONEY thus unjustly retained
still enforced and the ACTION remains AS A PERSONAL ONE against
him."
Smietanka Vs. Ind. Steel Co., 257 U.S. l., 42 S. Ct. L., 66 L.
Ed. 99, emphasis added.