Academia Stipend Repayment

I withdrew from graduate school on December 6th, and the semester ended December 5th. I received my stipend (direct deposited), as regular, for the entire month of December.

I got an e-mail from the institution's accounting person today stating that I owed money back to the institution because they overpaid me. Unknowingly, I was supposed to be paid a prorated amount for December, instead of the full stipend amount, now they're asking me to repay it.

I feel that I should not have to repay them because it was a clerical error on their behalf. The program director even apologized and said it was "his fault"; yet, I am still held responsible.

Do I have legal obligations to repay the amount they requested back?

----- Technically yes 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.