If one says visein tal umatar livracha after shmini atzeret, mitakenly thinking that it starts then, should he repeat shmonei esrei?
Answer:
There is some doubt as to whether Shemoneh Esrei should be repeated if vesein tal u-matar was mentioned after Shemini Atzeres, and the Mishnah Berurah (117:13; see also Biur Halacha) thus rules that one should daven again but stipulate that it should be as a nedavah (a voluntary prayer).
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