Question: Why aren’t raisins which the owner thought were not ready by twilight of Shabbos (bein hashmashos) muktzah just like a hammer or rock which one has no use for is muktzah as soon as Shabbos comes in?
A. The Shulchan Aruch (Orach Chaim 310:4) says that if dates or grapes are being dried and the owner puts them out of his mind as Shabbos comes in but by twilight (bein hashmashos) they were already fully dried and edible, even though the owner didn’t know at the time that it dried but after Shabbos started he was informed it was ready to be eaten at twilight. The Halacha is that it is permitted to be eaten and it is not muktzah though he seemingly mistakenly put them out of his mind as Shabbos came in.
B. Why not say a mistake is a mistake and it can’t be fixed just like any other muktzah item that you thought you had no use for as Shabbos came in and you might change your mind during Shabbos?
Answer: Look at Mishna Berura (17): Really he made no mistake in his mind, rather his mindset was that if as long as they are not ready they are out of his mind and once they are ready then he has them in mind to eat, he just wasn’t informed they were ready until after shabbos started but they were ready to be eaten as Shabbos started so they were never muktzah.