Throughout the eight days of Hanukkah, we insert Al Hanissim and Bimei Matityahu, which describes the miracle of the victory of the Jews over the Greeks: the many into the hands of the few, and the defiled into the hands of the pure. We mention this in the Amidah during the Modim beracha (blessing), to give gratitude to Hashem for all of the miracles that He performs daily for us, and in the beracha of Nodeh Lecha of the Birkat Hamazon. We do this for the entire eight days, but if a person, for whatever reason, forgot to insert Al Hanissim and Bimei Matityahu, they would not have to go back and repeat the Amidah, nor would they have to go back and repeat the Birkat Hamazon. To forget it is to say that you said Baruch ata Hashem (Blessed are You, Hashem) to conclude the beracha. Once you said Hashem’s name and you’re ready to say the beracha, a person does not go back. He doesn’t finish Lamdeini chukecha (Teach me Your statutes); he completes the beracha and proceeds as usual. If a person wishes, they can insert Al Hanissim and Bimei Matityahu at the end of the Amidah, before Oseh shalom bimromav (He who makes peace in His heights), or put Al Hanissim in as part of the Harachamans (post-meal supplications), but that’s voluntary; it’s not mandatory.
What if a person finished the Amidah and realized that he didn’t say Al Hanissim? He made a mistake: he thought he needed to go back and started the Amidah again on the premise that he needed to go back. Then he remembered in the middle of the Amidah that he doesn’t need to go back. He’s in the middle of the Amidah, so what does he do? He has to pause immediately. He does not finish the Amidah; he stops dead in his tracks. He stops the Amidah, says Baruch shem kevod malchuto le'olam va'ed (Blessed be the name of His glorious kingdom forever and ever) for the beracha levatala (blessing in vain) that he said, and then he ends. Why can’t he finish it in what we call tefillat nedava? A person is allowed to offer a voluntary prayer when they want, so why can’t he say in the middle of his prayer, “I change this to be a tefillat nedava,” and finish it so he doesn’t have a beracha levatala? The answer is that a person has to start and end the tefillah (prayer) under the same premise: either start and end as a chovah, obligatory, or start and end as a nedava. A person cannot begin the Amidah thinking he has to pray as a chovah and then change it midway into a nedava. The same could be said if a person thought he didn’t pray Mincha, for example, one day, and he got up to pray. In the middle of the Amidah, he realized, “Wait a minute, I prayed in the office already. I forgot.” He would also have to stop. He cannot change it midway in order to finish the Amidah, because he cannot start a tefillah as an obligation, thinking it’s an obligation, and then change it midway to nedava. The only exception is Tefillat Arvit. Hacham Ovadia writes that in Tefillat Arvit, since Arvit anyway by its core is optional, he can continue and change in the middle to be a tefillat nedava.