Don't worry. Appearing on a list of defaulters is more common than you think. The simple fact of returning a telephone, bank or utility bill may be reason for your data to be included in a national debtors file. It doesn't matter if the creditor made a mistake, if your card was stolen or if you stopped paying a bill because you were unhappy and wanted to file a claim.
When the nullity or abuse of a financial product has been claimed, and the debt is controversial, the banking entity CANNOT INCLUDE the plaintiff in a file of defaulters for debt derived from that product.