
ויאמר משה כה אמר ה' כחצת הלילה אני יוצא בתוך מצרים ... (יא-ד)
Citing the Gemara (Berachos 4a), Rashi explains that Moshe did not say that Hashem will come and kill the Egyptian firstborn "בחצת" at exactly midnight, but rather "כחצת" - around midnight, for "שמא יטעו אצטגניני פרעה" - Pharaoh’s astrologers will make a mistake in their calculations and say the plague did not come at midnight, accusing Moshe of being a fraud. Rashi further elaborates that the concern was perhaps according to their calculations, midnight will be a half-hour earlier than actual midnight, and when “their” midnight will come and go without anyone dying, they will dismiss Moshe’s prophecy as false.
R’ Simcha Maimon shlita explains that this is as the Rambam writes in his introduction to his commentary on the Mishnah: the hallmark of a true prophet is that every detail of his prophecy comes to pass, even the most minute. Thus, if Moshe’s timing would have been even slightly off, that would have indeed been enough of a reason for them to question his legitimacy. However, this concern seems difficult to understand. For although they might feel smug at the time, when midnight does arive, and the plague hits Egypt in full force, they will realize that they made a mistake, and that Moshe was correct all along!
Explains R’ Yosef Shalom Elyashiv zt”l, we see from here how careful we must be not to say or do anything that might result in a Chilul Hashem, even if it will be for only a split second, and even if it will be revealed afterward that there was a mistake! Anything which has the potential to possibly result in even the mere perception of Chilul Hashem, no matter how temporary or seemingly insignificant, must be avoided at all cost. On a related note, R’ Meir Stern shlita, Rosh Yeshivah of Yeshivah Gedola of Passaic, says that if people consider someone to be a Talmid Chacham, even if he knows that he is not, he must conduct himself in public as a Talmid Chacham would be expected to, due to the concern for Chilul Hashem - even though their expectations of him are mistaken! How careful must we be regarding Chilul Hashem!