One of the conditions of wudu (ablution) is washing all the parts in close 
sequence. Thus, if a person forgets to wash a limb required in wudu, or even a small part of it, and remembers it while performing wudu or immediately after finishing but while the washed parts are still wet, they may wash the part they forgot and the subsequent limbs only.
`Umar ibn Al-Khattab (may Allah be pleased with him) said that a person performed ablution and left a small part equal to the space of a nail (unwashed). The Messenger of Allah (peace be upon him) saw that and said: “Go back and perform wudu well. He then went back (performed ablution well) and offered the prayer.” (Muslim)
To know more about the conditions of wudu, you can watch this interesting video with Dr. Waleed Basyouni.