What is the ruling on someone who fasted six days of Shawwal after making up missed Ramadan fasts, but did not complete the sixth day's fast due to the end of the month and the beginning of the Day of Doubt, or due to not paying attention to the number of days in the month?

It is permissible to fast the six days of Shawwal before making up for missed Ramadan fasts. However, some scholars, including the esteemed Al-Uthaymeen, hold the view that it is not permissible except after making up for missed fasts. Therefore, whoever makes up for what they owe and then fasts the six days of Shawwal has done well. If a Muslim fasts six days with the intention that they are from Shawwal, but then it becomes clear that the last day was from Dhul-Qa'dah, they will receive the reward for fasting them according to their intention. This day will count as a make-up for the fasting missed in Shawwal, knowing that making up for the six days of Shawwal if they are missed is supererogatory. The default is that the month remains until it is known to have ended.