Berachos 23 - 29
Making up for Lost Prayer
If one forgot to say one of the regular prayer services he has an opportunity to "make it up" by praying the next Shmone Esrei service twice. This is true even if the makeup one is different in text from the one he missed, such as in the case of missing the weekday Mincha on Erev Shabbos which requires him to say the Shabbos Shmone Esrei twice during the Maariv service.
What happens, however, when someone forgot to say Yaaleh Veyavo in his Mincha Shmone Esrei on Rosh Chodesh? If he remembers before sunset he must repeat the Shmone Esrei because he had previously omitted that special reference to Rosh Chodesh. But if he does not remember until Maariv time an interesting question arises. It is no longer Rosh Chodesh and his makeup Shmone Esrei will not contain the Yaaleh Veyavo which he missed. It would therefore seem that there is nothing to be achieved with this repetition since he already has said a full Shmone Esrei minus Yaaleh Veyavo.
One opinion of the commentaries indeed contends that there is no point in doing a makeup prayer in such a case. Another position, however, is that the failure to say Yaaleh Veyavo at Mincha when it was required nullified that prayer altogether and it is as if he never said it, thus setting the stage for a makeup prayer at Maariv.
The Shulchan Aruch (Orach Chaim 108:11) rules that since there is a question as to which opinion to follow, one should say a makeup Shmone Esrei at Maariv, but should first declare that if that service is unnecessary it should be considered a voluntary prayer. Should Rosh Chodesh be on Friday, however, he cannot make this arrangement in the Shabbos Eve Maariv because voluntary prayers cannot be said on Shabbos.
The Nineteenth Blessing
Although we traditionally refer to the central portion of our prayer service as the "Shmone Esrei" because of the eighteen blessings it contains, the truth is that we actually say nineteen blessings.
In the days when Rabban Gamliel presided over the Sanhedrin in Yavneh he decided that the proliferation of heretics and informers amongst the Jewish people under Roman dominion presented such a threat to the physical and spiritual survival of the community that it was necessary to pray to Hashem to subdue or eliminate those dangerous elements. He therefore asked the sages if there was anyone amongst them capable of composing such a prayer which would become the nineteenth blessing.
The challenge was accepted by a sage named Shmuel Hakatan, who was famed not only for his great scholarship but for his piety and humility as well.
Perhaps the clue to why it was this particular sage who was best equipped to compose this prayer lies in the Biblical passage which he was accustomed to quote: "Rejoice not in the fall of your enemy" (Avos 4:24). Even in ideological confrontations there is the human tendency to seek victory over an adversary because of personal conceit. Only someone whose life's credo was avoiding such pleasure was qualified to compose a purely objective appeal for the defeat of elements only because they were enemies of Hashem and His people.