If you accidentally started saying weekday shmoneh esrei at Mincha on 1st day of Pesach, and you get past bareich aleinu, normally you just finish the current bracha then say yom tov mincha. But here, you lechora said v’sein tal umatar so do you have to start over?
This might make a nafka mina of whether you are allowed to say v’sein bracha 90 times on Yom Tov or is it considered hachana.