Kitzur Shulchan Aruch, Rabbi Shlomo Ganzfried
Chapter 76 : The Laws of the Prayers on Shabbos and Yom Tov
If you did not realize [your error] until you began saying the concluding berachos, (that is from Retzei [be pleased] and further) you should stop in mid-berachah, at the place you remembered, and begin (the berachah) of Shabbos or Yom Tov, and conclude (the prayer) in its (proper) order. If you did not realize until after you started to say: Yiheyu leratzon [may they be acceptable], you must start from the beginning of the Shemoneh Esrei.
If you erred in the Mussaf prayer and said the weekday berachah in the middle of Mussaf you must stop in the middle of the berachah in which you remembered your error, and begin the middle berachah of the Mussaf prayer, because for the Mussaf prayer there was never a halachic consideration to include all the middle berachos of the weekdays, but only the one berachah of Mussaf.
If one errs and begins the weekday Shemoneh Esrei on Shabbos it is a bad omen for him, and he should examine his deeds during the coming week and repent.