If one accidentally made a bracha on chodosh (and he is makpid on yoshon, due to the fact that it is a chiyuv d’oraysa to be makipid on yoshon, according to the vast majority of poskim), what should he do?
This is an excellent question.
If someone accidentally made a bracha and then he realized that the food is forbidden for him to eat, if the food is totally forbidden, then he should just say Boruch Shem etc. and he may not eat. However if I is only because of a minhag, and it is possible that the item is allowed to be eaten then he should taste a little bit in order that what he said shouldn’t be for nothing.
Therefore regarding the issur of not eating Chodosh; regarding gains that grew in Eretz Yisroel, where it is considered D’orayso according to everyone, he should just say Boruch Shem, (in actuality this is not too practical because it is usually winter wheat which is harvested after Pesach but was almost fully grown before, therefore considered yoshon after Pesach. Besides that the Rabbanut doesn’t allow such wheat to be used.) However in Chutz La’aretz, although there is good reason to be stringent about not eating chodosh, it issur isn’t totally clear, as there are poskim
Chazon Ish Demai 4-15, Shaarei Habracha 13-19, M:b Biur Halacha 485 D:H Af b’zman hazeh.