If a person recited the Beracha over Sefirat Ha’omer thinking the wrong number, for example, if he had in mind it was the fourth night when it was really the fifth night, but he caught his mistake in time and counted the correct number, he has fulfilled his obligation. Even though he had the wrong number in mind when he began reciting the Beracha, he nevertheless fulfilled the Misva and does not have to repeat the counting, since he ultimately counted the correct number. According to some authorities, however, if one had the wrong number in mind until after he completed the Beracha, he must repeat the counting even if he then realized his mistake and counted the correct number. In this case, according to this view, one must repeat the counting since he was thinking the wrong number throughout the entire recitation of the Beracha. Halacha, however, does not follow this view, and thus as long as one counted correctly, he has fulfilled his obligation regardless of his incorrect intention while reciting the Beracha.
The Halachic authorities also address the reverse case, where a person had the correct number in mind while reciting the Beracha, but then counted the wrong number. For example, a person on the fifth night of the Omer recited the Beracha with the intention of counting the fifth night, but then mistakenly thought that it was actually the fourth night, and thus counted four instead of five. The Shulhan Aruch, surprisingly, rules that a person fulfills his obligation in such a case, since he had the correct counting in mind when he recited the Beracha. Later authorities, however, questioned this ruling, and wondered how the Shulhan Aruch could consider a mistaken counting valid on the basis of the person’s intent while reciting the Beracha. Therefore, in such a situation one should count again, correctly, but without reciting a Beracha.
If a person counted the wrong day – such as if he recited "Hayom Arba’a Yamim" ("Today is the fourth day") on the fifth night – but he then immediately corrected his mistake and recited "Hamisha Yamim" ("the fifth day"), he has fulfilled his obligation. Since he corrected his mistake within a few moments – the period known as "Toch Kedeh Dibur" – he has fulfilled his obligation and does not have to repeat the counting. In the reverse case, where somebody counted correctly and then immediately changed his mind and counted the wrong number, he has also fulfilled his obligation. Since he did, after all, count the correct number, he has fulfilled the Misva and does not have to repeat the counting.
There is a debate among the Halachic authorities concerning the case of one who recited the Beracha over Sefirat Ha’omer and then paused silently for a few moments – longer than the period of "Toch Kedeh Dibur" – before counting. For example, a person did not have the Siddur opened to the correct page or did not find the place on the page before reciting the Beracha, and then after reciting the Beracha he had to scramble to find the place. According to the Magen Abraham (Rabbi Abraham Gombiner, Poland, 1637-1682), one does not fulfill the Misva in this case, since there was an extended pause in between the Beracha and the counting. Others, however, maintain that as long as one did not speak in between the Beracha and the counting, the Misva has been fulfilled. The Mishna Berura (Rabbi Yisrael Meir Kagan of Radin, 1839-1933), in Siman 206 (Se’if Katan 12), indicates that Halacha follows the lenient view in this regard, that one fulfills the Misva even if there was an extended pause after the Beracha. However, needless to say, it is proper to ensure not to recite the Beracha until one has the Siddur open or the calendar in front of him and is prepared to count immediately after reciting the Beracha, in order to avoid this Halachic dispute.
Summary: If one had the wrong counting in mind while reciting the Beracha over Sefirat Ha’omer, but in the end counted correctly, he has fulfilled the Misva. If one had the correct counting in mind while reciting the Beracha but ultimately counted incorrectly, he must count again, correctly, but without a Beracha. If one counted incorrectly and then immediately corrected his mistake and counted the proper number, he has fulfilled the Misva. If one counted correctly and then immediately counted the wrong number, he nevertheless fulfilled the Misva since he initially counted the correct day. One must ensure to be prepared to count immediately after reciting the Beracha. Nevertheless, if one paused in between the Beracha and the counting, he has fulfilled the Misva, as long as he did not speak in the interim.
