If it was the Sawm (fasting) of Ramazan and the couple deliberately involved in sexual intercourse, then both the husband and wife have to compensate for it (kaffarh). And, if the fasting was outside Ramazan then there is only qaza (making up missed fast) not kaffarh.
The kaffarh means to fast consecutively for two months without a gap; only a woman is permitted to pause during her menstruation periods. And, one who is not able to fast for illness or weakness, then he or she has to feed sixty poor people.
By ?mistake? if you mean something else then write it clearly.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband