Overpayments happen more often in SSI cases because there is a limit on how much money you can make. Often this is because you worked and earned more money than SSA allows, but it can just as easily be a gift of money from family or friends, a landlord who doesn’t make you pay rent, or receiving extra money for food. This can all count as income and SSA deducts money from your SSI check accordingly. If SSA decides that you should be getting less money, they will come after you for the overpayment.
There is no time limit on when SSA can come after you for overpayment. SSA tries to take care of them quickly, but there is a backlog and in some cases the overpayment can go on for many years. This means that several years into receiving benefits you can get a letter telling you that you owe SSA tens of thousands of dollars.
First look at the numbers to make sure they make sense. If SSA is in error and you really were owed the money, let them know by filing a Request for Reconsideration (http://www.ssa.gov/online/ssa-561.pdf). This is not for cases where the overpayment is not your fault and you would not have known that you were overpaid. Nor is it for cases where you cannot afford to repay SSA. It is only for cases where you were not overpaid and SSA has incorrect information somewhere.
If the overpayment is valid, but you cannot afford to pay it back, you need to file a Request for Waiver. (http://www.ssa.gov/online/ssa-632.pdf). This is in cases where the overpayment is not your fault and there is no way you would have known that you were being overpaid. SSA will work with you in order to figure out a payment plan you can afford. Filing a waiver and trying to work out a payment plan is your best chance for dealing with the overpayment. Try to figure out if you can afford to pay anything per month and make an offer to SSA. They will work with you to resolve the overpayment without bankrupting you.