Someone said in Ya'aleh v'Yavo of Mincha on Hoshanna Rabba, just before the onset of Shemini Atzeres, SA instead of Sukkos. What is the halacha? I could not find anyone who discusses it directly. But acharei ha'iyun there are two tzedadim to be lenient:
1. The shittah that so long as you said YvY it is considered mei'ein ha'meora, even if you mentioned the wrong mo'ed.
2. That the Mincha was after plag, and therefore at a time suitable to be mekabbel SA anyway.