It can knock you down pretty far depending on how much you owe. $300 is probably not going kill your credit outright if it's very good, but if it's average, you could see some serious problems including rate hikes to your credit cards and trouble qualifying for loans or possibly even housing or jobs. In which case, they wrongfully damaged you and you can take them to court for a small pile of money and have it taken off your credit. It's your responsibility to show that you were wrongfully affected by their actions(so save any documents on such things)but they must show that they properly put this on your credit report and some places are harsher than others for that. Also you can dispute it in the meantime although that has mixed results generally if they have your signature on a sheet of paper. The credit bureau's don't take a lot of time to handle disputes and tend to take the word of the businesses over the individual.