saurabhindian,
If a client fails the DC, that means their Courtesan believes they have a STD (sexually transmitted disease). After failing a DC, another experienced Courtesan may perform another verifying DC to make as certain as possible the client does in fact have a STD. If both Courtesans believe the client has a STD, they may not have a sex party, and are advised to seek a medical doctor for further evaluation and treatment.