On the days of Tashriq (11th, 12th, and 13th of Dhul-Hijjah), the three الجمرات must be stoned in a specific sequence: first the smallest Jamarah (al-Sughra/al-Ula), then the middle Jamarah (al-Wusta), then the largest Jamarah (al-Kubra/Jamrat al-Aqabah). On the 10th of Dhul-Hijjah, only Jamrat al-Aqabah is stoned, so the sequence issue does not arise.
The majority of scholars (Maliki, Shafi'i, Hanbali) hold that the correct sequence is واجب (واجب) or even a condition (shart) for the validity of the stoning. If a الحاج stones them in the wrong order — for example, starting with the largest or middle Jamarah — they must repeat the stoning in the correct order. الشيخ ابن عثيمين explained that النبي (صلى الله عليه وسلم) stoned them in sequence and said 'Take your rites from me,' making his order binding.
The Hanafi school takes a more lenient position, holding that the sequence is السنة (مستحب) but not a condition of validity. According to this view, stoning in the wrong order is valid but contrary to the السنة, and no penalty or repetition is required. However, the الحاج misses the reward of following the prophetic sequence. الشيخ ابن باز followed the majority position and advised that if a الحاج discovers they have stoned in the wrong order and the day's stoning time has not ended, they should repeat the stoning in the correct sequence from the point where the order was broken.