As everyone knows, Simchas Torah is a day of lively dancing. It is also the day when we begin saying mashiv haruach umorid hagoshem. But we do not switch over to vesein tal umotor livrocha until weeks later. This point can be confusing. After all, if we praise Hashem for causing the wind to blow and the rain to fall, why not request rain? The answer, that time is given for people to return home from their pilgrimage to Yerushalayim, is somehow easily forgotten. It is not unusual for someone to accidentally say vesein tal umotor livrocha immediately after Simchas Torah.
When this question was presented to Rav Yosef Shalom Elyashiv, he replied that Shemoneh Esrei need not be repeated. “If someone requested tal umotor in the days between Simchas Torah and when one should ask for rain, he need not repeat Shemoneh Esrei. By rights, we should all request rain from Simchas Torah, since that is when we begin to say mashiv haruach umorid hagoshem. Nevertheless, one who erred once in this manner should say a tefillas nedovah. When he does so, he should recite an additional request as well, during shomea tefillah” (Mevakshei Torah; Shu”t Avnei Yashpeh).