Is it obligatory to make up a fast after drinking, thinking that Fajr had begun, only to then realize that it had not?

If it becomes apparent that you drank after the dawn broke, this issue is a matter of scholarly disagreement. The majority of scholars hold that qada' (making up the fast) is obligatory, while some scholars—such as Ibn Taymiyyah—are of the opinion that qada' is not obligatory in this case. Both opinions are narrated from 'Umar, may Allah be pleased with him, and making up the fast in this instance is a good precautionary measure.