From Cato Institute:

Several years ago, Nick Bronsozian was charged with possession of an unregistered machinegun under a tax law statute. The provision in question, 26 U.S.C. § 5861(d), says that in order to have a machinegun registered, a tax must be paid on it. Simple enough, right? Bronsozian didn’t pay his tax. Case closed. That’s what the government argued anyway, but the situation is more complicated than that.
A subsequently enacted law, 18 U.S.C. § 922(o), prevents the government from registering and accepting tax payments on new machineguns. So Bronsozian was charged and convicted of a felony for not paying a tax that the government would not allow him to pay. If that strikes you as odd, it’s probably because you’ve read the Constitution.