Say you were terminated from a company in April of this year. Business was slow since the beginning of the year. Your dismissal was processed through HR, but the info was not relayed to the corporate payroll office. Policy fro checks for workers were mailed to employees. Your checks continued the next four weekly paydays. It wasn't until the monthly payroll reports were sent to your supervisor that the error was detected. You refuse to return the checks, what action could the company take?

They can and will easily sue you for the over payment. When you first got an extra check you were aware of their error. Their error does not mean you get to keep any money you did not earn.

Prosecute you for theft.

Yes, while it was there mistake to send it. It was yours to keep it.

Each state has its own criminal law, but this could well be considered a fraud, since you did not notify them of the mistake when you got first check.

Also I would assume you were not collecting unemployment when you got the checks, that could be even a more serous criminal offense.