If someone by mistake didn’t say “atah yetzartah” and instead davened a regular musaf on Shabbos Rosh Chdoesh, would they be yotzei bedieved, since they did say “musafim kehilchasam”?
Answer:
No, if “ata yetzarta” is forgotten, and one has already finished the Mussaf davening, one must repeat Mussaf.
The reason for this is that this involves a significant change in the text of the berachah.
Best wishes.
Sources:
Chayei Adam 28:4; Mishnah Berurah 268:16.
