Question: Why does the concept of “toch kidei dibur” work to correct oneself if he says the wrong day of the Omer but not if he mentions Shabbos instead of Yom Tov in his shemone esray?
A. “Toch kidei dibur” is the concept of realizing one made a mistake and immediately correcting himself within a certain short amount of time which is the amount of time it takes for a student to greet his rebbe saying, “Shalom aleichem rebbe umoreh.”
B. The Mishna Berura (Orach Chaim 488:6:32) says “they further write that if one makes a mistake and says ‘today is the fourth day of the omer’ and toch kidei dibur remembers it is the fifth day, it is enough to just finish ‘fifth of the Omer’ and he fulfills the mitzvah even if he didn’t say ‘today is the fifth day’ since it was still within the allotted time of correction.
C. If one on Yom Tov says “mikadesh hashabbos” instead of “mikadesh Yisrael vihazmanim,” the Mishna Berura (487:3) says one must go back and say “mikadesh Yisrael vihazmanim” and it’s not enough just to correct oneself toch kidei dibur and say “Yisrael vihazmanim” after concluding “mikadesh hashabbos.”
D. When Yom Tov falls out on Shabbos we say in our shemone esray “mikadesh hashabbos Yisrael vihazmanim.”
Answer: Rav Shlomo Zalman Auerbach explains that it is not recognizable that you are correcting the mistake you made since that is just what you say on Shabbos Yom Tov but by the Omer it does look like he is correcting himself since one does not count twice in one day. (See Dirshu Mishna Berura there note 68.) [/exapnd]