Do I owe former company for payroll error?

My job closed down and I was sent 2 checks after I left. The company calls back a week later and says that they made a mistake and now they tell me I need to cut them a check for the extra amount. The checks were both signed and were completely legitamate. They came a week apart and I cashed both of them. Aside from the ethical problem of keeping the money, what does the law say?

you do need to repay if you agree it is an error; you can however argue hardship in having to pay all or any substantial portion--saying that it was their mistake after all, and you spent it w/o realizing it was in error, and paying back now would be difficult...they often will not sue or take any further court action in these cases but will try to work something out. They may even settle on a partial amount.

I am also quite worried about a similar, yet slightly larger, problem. I left my job to travel, upon return I have discovered that my old work place did not take me off payroll and I now owe the company a substantial amount of money, a lot of which I have spent unknowingly. Considering the magnitude of the sum of money, is there a chance I will encounter legal problems?