I am a former Fairfax County resident. I received a notice to appear in Court, being sued by the County for two tax years during which I was a resident of a different state. I have proven residency in another state, including documentation taken from the County land records and they continue to pursue. Do you know of others that this has happened to?

Assuming you're referring to movable property such as car or boat, I believe you're obligated to inform the tax administration office that you've moved out of state and would no longer be subject to the personal property tax. I don't see how they would know that you've transferred the property outside of a taxable jurisdiction, even if you re-registered with another state's DMV. Check out the regs on the county's tax administration website. -§

from the fairfax county website

A vehicle is subject to tax if it is normally garaged or parked in the County, even if registered in another state. Tax on motor vehicles is prorated for the portion of the year it is located in the County unless the vehicle is moved to a non-prorating Virginia locality.

If you "did" become a "resident" of another state, then show up to court with
1) Utility bills from another state
2) a Voter registration card from another state.

If FFX can prove you were a resident of ffx or at least the car was garaged here - a library card that was used during this period, a utility bill in your name, etc, you are screwed. Good luck.