Among the mistakes that are made when cutting the hair or shaving the head are the following: 

1 : Some people shave part of the head completely with a razor, and leave the rest. I have seen that with my own eyes. I saw a man doing saai between al-Safa and al-Marwah, and he had shaved half of his head completely and left the rest of his hair. I grabbed him and said to him, “Why did you do that?” He said, “I did it because I want to do Umrah twice, so I shaved half for the first Umrah and left half for this Umrah.” This is ignorance and misguidance that none of the scholars ever suggested. 

2 : Some people, when they want to exit ihraam of Umrah, cut a few hairs on their head, on one side only. This is contrary to the apparent meaning of the verse in which Allaah says (interpretation of the meaning): 

“…(some) having your heads shaved, and (some) having your head hair cut short…”
[al-Fath 48:27] 

The shortening of the hair should have an obvious effect on the head. It is known that cutting one or two or three hairs does not have any effect and it is not apparent that the pilgrim who has done Umrah has in fact shortened his hair. So he is going against the apparent meaning of the verse. 

The remedy for these two mistakes is for the one who wants to shave his head to shave all of it, and to shorten the hair all over the head if he wants to cut it, and not to cut just one or two hairs. 

3 : There are some people who, when they have finished saai, if they cannot find anyone to shave their heads or cut their hair, they go back to their houses and exit ihraam, and put on regular clothes, then they shave their heads or cut their hair after that. This is a serious mistake, because a person cannot exit ihraam without shaving his head or cutting his hair, as the Prophet (peace and blessings of Allaah be upon him) said, when during the Farewell Pilgrimage he commanded those of his companions who had not brought a hadiy (sacrificial animal) to make it Umrah, and he said, “Let him cut his hair then exit ihraam.” Narrated by al-Bukhaari, 1691; Muslim, 1229.  This indicates that one cannot exit ihraam until after cutting the hair. 

Based on this, if a pilgrim finishes saai and cannot find someone to cut his hair, he should remain in ihraam until he shaves his head or cuts his hair; it is not permissible for him to exit ihraam before that. If we assume that a person did that out of ignorance and exited ihraam before shaving or cutting his hair, thinking that it was permissible to do so, there is no blame on him because of his ignorance, but when he finds out about that he should take off his regular clothes and put on his ihraam garments, because it is not permissible to continue exiting ihraam when he knows that it is not permissible. Then when he has shaved his head or cut his hair, he may exit ihraam.”