As an ordinary person may not escape the "forget". Many activities that have been planned but not implemented because of the forgotten factor.
So what if we forget the wudhu or wash a small part of the ablution. But immediately remember it after completion ablution, whether to re-repeat ablution back?
A sequence or tartib is a requirement for the ablution of wudhu. If someone forgets to wash one of the members of the ablution (though small) then he just washes away the forgotten part of it.
But if he forgets to wash the ablution after the water has dried up in the prayer or after the prayer, then he should take ablution again. As Allah has set forth, then he returns to prayer perfectly in order to realize tartib in this state.
Allah Almighty obliges to wash all parts of ablution. Who left (though small), then as if leaving everything.
Like the hadith of Umar bin Al-Khattab radiyallahu 'anhu, "Rasulallah shalallahu' alaihi wa sallam saw the man performing the ablution but he did not wash a piece of his toenails. So he told him to repeat his wudhunya saying, "Repeat your wudhum," (Ibn Majah in his sunan, Imam Ahmad, Abu Dawud, etc.).