I’m not quite sure that it’s that simple. Someone can correct me if I’m wrong. Certainly, if it’s a venial sin then you don’t need to confess it in a future confession, and strictly speaking you are not even required to confess every venial sin in order to receive absolution.** As for a forgotten mortal sin, it seems to me that one should return to confession for that. Should one receive Communion before they make it back to confession? I don’t know the answer to that. Imperial Guard, can you back up that all mortal sins are forgiven in confession though they were inadvertently not confessed?**