In the context of improving your credit score, I use the term "bad debts" to refer to any item where the creditor, for whatever reason, cannot substantiate the amount due and owing. This also includes an account which has been mishandled by the creditor.
Credit reports can be littered with all kinds of wrong information from identity theft; incorrect credit history and personal information; accounts which are not yours; improper charges, calculations and write-offs; "joint" accounts that are not joint; and, my personal favorite, denial of credit because information in the credit report was wrong to begin with.
Recently, Fair Isaac Corporation recognized that medical accounts in collection need to be reported and scored differently, because medical billing companies were failing to communicate with the consumer. A consumer should not be penalized for a collection lawsuit filed by a medical billing company, when, by way of example, no bill had been sent.
Credit card companies have been notorious for selling their old accounts, which are then re-packaged and resold again and again to various collection agencies. The original credit card agreement and statement of account no longer exist. Often accounts are so stale that the statute of limitations bars recovery of whatever may have been due, if anything.
The Fair Credit Reporting Act sets forth specific regulations regarding credit reporting. You need to assert your rights to protect yourself and to clean up your credit reports. Only a trained professional, such as an attorney, will have this knowledge and resources to assist you.
Illegitimi non carborundum.