One year after closing, the seller's attorney found an error credit paid to buyer. Should the buyer pay back?

This really comes down to the amount credited in error. Without more information about the erroneous credit, pursuing the refund may be more costly than recovering that amount. Contact the attorney that represented you at closing for clarification. If you proceeded without an attorney for this transaction, then it becomes a little more problematic - you can always contact the sellers attorney for clarification, but he or she may not be responsive.

Not enough information is provided. What was the credit, what was the basis of the error, what does your purchase contract say, commercial or residential transaction, etc., etc.? In addition there may a merger argument, since this has closed and likely the deed has been delivered. You should talk to the attorney who represented you in the transaction. If you did not have one, you may want to consider at least discussing the situation with a real estate attorney (and this would be an excellent example of why you always should hire one when you purchase or sell real estate).

I agree with my colleagues. Moreover, most real estate contracts state that all prorations are final. Both parties would have to admit to the error and it would have to be material, among other things. If you had an attorney for closing and he or she knew or should have known of the error too, then that attorney should be consulted as well.