No kavono [intent] in Avos
The MB in Biur Halocho (siman 101) writes that a person who said Shmone Esrei and had no kavono [intent] in the bracha of Avos, should “save” his Shmone Esrei by listening and fulfilling his obligation with the chazoras hashatz. What is the Rav’s opinion in this matter?
If one has already completed his Shmone Esrei he does not go back. If one is in the middle of Avos he may go back to “elokeinu velokei avoseinu” and continue from there. Even one who has completed the bracha of Avos can pause at that point and have in mind the meaning of birchas Avos, and in this way “fix” up his missed kavana. This procedure is said in the name of the Chazon Ish.
The case the Biur Halacha refers to is one who completed Avos and did not have intent, to which the Biur Halacha writes it is problematic for him to continue, so he should wait and listen to the first bracha from the chazzan and then continue on his own.
According to the Chazon Ish above, he can stop and have in tent up until the point he is holding and then continue.
Practically the custom does not follow the Biur Halacha, rather the Chazon Ish. This is due to the concern of making a long pause in the middle of one’s shmone esrei, and also not having in mind when the chazan says his repetition as we are not accustomed to fulfilling our obligation with others for Tefilla, See Yabia Omer 3:10.
See Orchos Rabeinu, Vol. 1, pg. 59, Halichos Shlomo 8:9.