Kosher certificate – prohibited foods?

I didn't understand the question. Isn't it possible that the rabbi or the overseer is wrong? Perhaps you are asking what the ruling is if the overseer was wrong, whether the food was forbidden. Simply put, yes – except that it was forced or accidental. What's more, if it can be checked, then you are not forced but at most accidental, and then there is a transgression. And beyond that, with regard to forbidden foods, Chazal and the Rishonim claim that there is a foolishness of the heart in eating them ("and you have become ignorant of them"), and therefore are concerned even if there is no prohibition in this on the formal halakhic level.