Praise be to Allah.
Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
The muhrim should not comb his hair, because what the pilgrim should do is to look dusty and unkempt. There is nothing wrong with him washing his hair, but combing may cause hair to fall out. But if some of the muhrim’s hair falls out without him intending that, because of his scratching or rubbing his head and the like, then there is no sin on him in this case, because he did not deliberately intend to remove those hairs. It should be noted that with regard to all the things that are forbidden in ihraam, if a person does not intend to do them deliberately but he happens to do them by mistake or out of forgetfulness, there is no sin on him in this case, because Allaah says in His Book (interpretation of the meaning):
“And there is no sin on you concerning that in which you made a mistake, except in regard to what your hearts deliberately intend. And Allaah is Ever Oft-Forgiving, Most Merciful” [al-Ahzaab 33:5]
“Our Lord! Punish us not if we forget or fall into error” [al-Baqarah 2:286]
And Allaah has said, “I will do that.”