Am I liable for bank's mistake? Please help!

[oaknet]

I am a student and I have a checking account at Washington Mutual in California. My mom makes monthly deposit into my account for a total of 1,200 each month. During one month, she accidentally wrote Twelve Thousand instead of Twelve Hundred in the written part of the check (still show 1,200 in numeric part). The bank credit me with 1,200 but later took out 12,000 from my account when the check bounced. I brought the issue to the bank and they promised to fix it. Couple days later I got a 12,000 credit and a 1,200 debit (to balance it out) to my account. To my surprise, they also gave me another credit of 10,800. I happily used this money to pay off my credit card debt. Couple month later, they found the mistake and took 10,800 from my account. So now I have a negative 10,000 balance on my account. A lady called me from Washington Mutual corporate investigation and told me that they can collect the amount from me even though it was a bank mistake, she then offer me a very nice 2 year payment plan interest free and wants me to sign the promissionary note.

Here is my question before I sign the note, am I legally liable for this debt even though it was their mistake? If I don't pay off the debt, what can they do to me?
Please help, I am totally lost.
Thanks for your time