One who is unsure whether he neglected to count one day may continue counting with a blessing, because we worry about the opinion that says that one cannot continue counting with a blessing only when one is certain that he missed a day.
Similarly, one who forgets to count at night but remembers and counts during the course of the day counts with a blessing on all subsequent nights. Even though there are those who hold that one does not discharge one’s obligation by counting during the day, [we follow] those who maintain that, be’di’avad, one who counts during the day fulfills the mitzvah.
The law of a young boy who becomes bar mitzvah in the middle of the sefirah period is questionable. Some poskim hold that even if the boy is careful to count every day, he cannot continue counting with a blessing, because the days he counted before becoming bar mitzvah are not considered one continuum with the days he will count after reaching adulthood. According to most poskim, however, if the boy is careful to count every day before his bar mitzvah, his counting is considered complete and he may continue counting with a blessing. This is the prevalent custom.
A gentile who converts to Judaism during the sefirah period counts without a blessing, because he did not count at all before his conversion.
According to most poskim, even if one remembers to count only during the twilight period of the day, he may count the following nights with a blessing. There are two reasons to doubt that one has fulfilled his obligation in such a case: Some authorities hold that counting during the day is invalid. And even if we say that one discharges his obligation by counting during the day, it is uncertain whether twilight is part of the day or the beginning of the next night. Nevertheless, since it is not definite that the person missed a day, he can rely on the majority of poskim who hold that each day is a separate mitzvah and continue counting with a blessing. Furthermore, according to Rabbeinu Tam, our definition of twilight is definitely daytime, and the Shulchan Aruch (O.C. 261:2) codifies this as halachah. Now, even though we do not follow this ruling, we may use it in a case of doubt in order to rule leniently. There are those who rule strictly, such as K.H.C. 489:83, but as we stated above, most poskim hold that [one who counts during twilight] may continue counting with a blessing. This is found in Sho’el U’Meishiv, Yabi’a Omer (vol. 4, O.C. 43), and Hilchot Chag BeChag 6:7.