What are the rulings regarding absentmindedness (sahw) during wudu (ablution)?

Ablution (Wudu') has obligatory acts (fara'id) and sunnah acts. Forgetfulness during it can pertain either to the obligatory acts or the sunnah acts. If the forgetfulness relates to the agreed-upon obligatory acts of Wudu' (washing the face, washing the hands up to the elbows, wiping the head, washing the feet), then the Wudu' is invalid, and the forgotten obligatory act must be performed. If continuity (muwalat) is lost, then the Wudu' must be redone, and the prayer too if it has already been performed. The proof for this is the hadith of the man who prayed, and there was a dry spot on his foot that water had not reached. The Prophet, peace and blessings be upon him, commanded him to repeat his Wudu' and his prayer. However, if one forgets a sunnah act of Wudu', then their Wudu' is valid. And if one doubts the number of times an limb has been washed, they should assume the lesser number.