That was my first thought.
My second was this:
An armed thug breaks into your home. You own a registered firearm, get the drop on him and capture him. His firearm was illegal.
The city you are in is going through a budget crisis. When the city cops report that a burglar was arrested in your home with an illegal firearm, they seize your house and sell it.
Toss his gun in your annoying neighbor's yard.