Shaykh Ibn ‘Uthaymeen said:
If he does one of the things that are forbidden in ihraam out of forgetfulness or ignorance, there is no blame on him, but he must correct himself as soon as he remembers or finds out, so it is essential to remind people and to teach those who are ignorant.
For example: if a man forgets and puts on regular clothes when he is in ihraam, he does not have to do anything, but as soon as he remembers he has to take off these clothes. Similarly if he forgets and keeps his pants on, then he remembers after he has formed the intention and started to recite the Talbiyah, he has to take off his pants immediately and he does not have to do anything else. And if he was ignorant of the ruling, he does not have to anything, such as if he wore a t-shirt in which there was no stitching, thinking that the muhrim can wear anything that has no stitching, he does not have to do anything. But once it is explained to him that a t-shirt, even if it has no stitching, is a kind of clothing which he is forbidden to wear, then he has to take it off.
The basic principle concerning that is that if a person does any of the things that are forbidden in ihraam out of forgetfulness or ignorance, or because he is forced to do them, there is no blame on him, because Allaah says (interpretation of the meaning):
“Our Lord! Punish us not if we forget or fall into error”
and Allaah has said, “I will do that.”
And Allaah says (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”
And Allaah says concerning hunting in particular (interpretation of the meaning):
“…and whosoever of you kills it intentionally …”
It makes no difference whether the thing that is forbidden in ihraam has to do with dress, perfume etc, or hunting game, or shaving the hair on one's head, etc. Some scholars differentiate between these things, but the correct view is that there is no difference, because these are things that are forbidden but for which a person may be excused if he does them out of forgetfulness or ignorance, or because he is forced to do them.