325. Forgetting Dew and Rain

Tefillah u’Birkas Kohanim 10:8

If one makes a mistake in the winter and neglects to recite Morid hageshem (that God causes rain to fall) or Morid hatal (that God causes dew to fall), he must go back to the beginning. If he mentioned dew (v’sein tal), however, he need not repeat Shemoneh Esrei. If one makes a mistake in the summer and recites Morid hageshem, he must go back to the beginning. If he did not mention dew, he need not repeat Shemoneh Esrei. This is because dew is never withheld and there is no need for us to request it.

Tefillah u’Birkas Kohanim 10:9

Let’s say that one forgot to request rain in the bracha for prosperity. If he realizes his mistake before he reaches the bracha of Shomeiah tefillah (that God hears prayer), he should request rain there. If he realized it after reciting Shomeiah tefillah, he must go back to the bracha for prosperity. If he does not realize his error until after he finishes Shemoneh Esrei, he must go back to the beginning and daven again.