If a person mistakenly began reciting "Ata Honen" – the weekday text of the Amida – when reciting the Amida on Shabbat, he should finish the Beracha before switching over to the proper text. Meaning, instead of immediately stopping and reciting "Ata Kidashta," "Yishmah Moshe" or "Ata Ehad," he should continue reciting "Ata Honen" until "Honen Ha’da’at," and then proceed to the proper text for Shabbat.
However, Hacham Ovadia Yosef rules in Hazon Ovadia (p. 349; listen to audio recording for precise citation) that this is not the case if the Hazan makes this mistake. If the Hazan began reciting "Ata Honen" in the repetition of the Amida on Shabbat, and then realizes his mistake, he should immediately discontinue the recitation of this Beracha and switch to the proper text for Shabbat. The reason, Hacham Ovadia explains, is because the Hazan should not overburden the congregation with the recitation of additional Beracha. Even though this additional recitation would take only several moments, nevertheless, even this extra burden must be avoided. Moreover, a Hazan’s reciting a weekday Beracha in the repetition of the Amida on Shabbat would cause some commotion in the synagogue, which we certainly wish to avoid. Therefore, as Hacham Ovadia cites from the Hikreh Leb and other Halachic authorities, the Hazan in this case should immediately discontinue the recitation of "Ata Honen" and proceed to the proper text.
As discussed in a previous edition of "Daily Halacha," Maran (Orah Haim 268:4) rules that if a person mistakenly recited the complete weekday Amida on Shabbat, he has not fulfilled his obligation unless he mentioned Shabbat somewhere in the Amida. If he happened to make mention of Shabbat in the Amida, he has fulfilled his obligation and does not have to recite the correct Amida, even though he recited the weekday Amida. On the basis of this Halacha, Hacham Ben Sion Abba Shaul (Israel, 1923-1998) arrived at an intriguing conclusion. He ruled that if a person mistakenly recited the weekday Amida on Shabbat and realized his mistake when he reached the Beracha of "Reseh," he should recite "Ya’aleh Ve’yabo" after "Reseh," just as we do on Rosh Hodesh and Yom Tob. Although we do not normally recite "Ya’aleh Ve’yabo" for Shabbat, in this situation one may recite "Ya’aleh Ve’yabo" and say the insert of "Et Yom Ha’Shabbat Ha’zeh." Hacham Ben Sion explains that initially, the Sages were going to have us recite the standard weekday Amida on Shabbat, and it was only in order not to overburden us with a lengthy prayer service that they introduced a special, shorter prayer text for Shabbat. Presumably, Hacham Ben Sion reasons, when the Sages considered having us recite the standard weekday Amida on Shabbat, they intended for us to add "Ya’aleh Ve’yabo" in the Amida as we do on Rosh Hodesh and Yom Tob, so that we would mention Shabbat somewhere in the Amida. Therefore, in a case where one mistakenly recited the weekday Amida, he may follow this initial format, adding "Ya’aleh Ve’yabo" in "Reseh."
A somewhat similar ruling was issuing by Rav Shlomo Zalman Auerbach (Jerusalem, 1910-1995) concerning the case of a person who has only a weekday Siddur and does not know the Shabbat Amida by heart. Rav Shlomo Zalman ruled that since the person in this case can only recite the weekday Amida, he should recite that Amida and add "Ya’aleh Ve’yabo," reciting "Yom Ha’Shabbat Ha’zeh."
Summary: If a person mistakenly began reciting "Ata Honen" in the Amida on Shabbat, he should complete the Beracha before proceeding to the Shabbat text. However, if the Hazan made this mistake, he should switch to the Shabbat text immediately upon realizing his mistake, without completing the Beracha. If a person realizes on Shabbat as he reaches "Reseh" that he had recited the weekday Amida, he should recite "Ya’aleh Ve’yabo" and add "Yom Ha’Shabbat Ha’zeh," and can thereby fulfill his obligation. Likewise, if a person has only a weekday Siddur and does not know the Shabbat Amida by heart, he should recite the weekday Amida and add "Ya’aleh Ve’yabo," reciting "Yom Ha’Shabbat Ha’zeh."