I found this while playing with the foxycart example store.
1) Add an item to the cart.
2) Chose a country other than USA
3) Type in random stuff for all other required address fields.
4) click "confirm your order"
5) edit/delete your State field.
It now no longer accepts anything other than an actual State in this field.
This is actually a known issue. The FoxyCart example store (shamefully enough) is on an older version of FoxyCart. (We're so busy improving our system that we sometimes neglect our demos.)
If you see this behavior in your own test store please do let us know. Also worth noting is that the country is auto-populated based on the customer's IP address, so this isn't an issue most users would typically see in the first place.