Knowing the answer to this type of question is important because a quick decision is needed, and sometimes the rav is not present. It is difficult to choose from among the various opinions on the matter, and there are distinctions about which poskim differ. We will try to explain the basic approaches and then offer our suggestions.
The Beit Yosef[2] relates the following incident discussed by the Avudraham: On Rosh Chodesh of Tevet, the sefer Torah for the Chanuka reading was incorrectly opened before the one for Rosh Chodesh, and the problem was corrected after the oleh made his opening beracha. The Avudraham cited an opinion that the oleh should have made another beracha because of the delay caused by the rolling of the sefer Torah to the correct place. This position further compared the case to one who made a beracha on one food but ended up eating another – he is required to make a new beracha. Another authority disagreed with the first premise and, regarding the second, said that the beracha applies to all passages in the sefer Torah. Therefore, he says, making a new beracha is improper.
The Beit Yosef concludes that since we require a new beracha on the food he had not intended to eat even though it was before him at the time of the first beracha,[3] the misled oleh also must make a new beracha. Although he includes both aforementioned opinions in the Shulchan Aruch,[4] he favors the one that mandates another beracha (without repeating the introductory passages of Barchu[5]). Nevertheless, recent Sephardic poskim[6] conclude, contrary to the Shulchan Aruch’s ruling, that since there is a doubt whether or not to repeat the beracha, one should refrain from reciting it.
Ashkenazic poskim generally require a new beracha in such a case, but several caveats make the application of this rule somewhat rare. Most classical poskim rule that the matter depends on the oleh’s intention during the beracha. Since most olim do not think very deeply about what they are making a beracha over, the poskim must assess the oleh’s implicit intent. If the oleh becomes aware of the mistake before the end of the beracha, he need not make a new beracha.[7] (Rolling the Torah without the oleh realizing would not help.) The Mishna Berura[8] rules that all sections of the Torah that are visible to the oleh are included in his beracha. (The Sha’arei Ephrayim[9] requires that both the correct and intended texts be in the same column). Thus, the most common mistakes that require a new beracha occur in the first aliya; likely scenarios include instances where the wrong Torah was taken out, the Torah was rolled inaccurately, or the correct opening column was moved during the previous hagba.
The Pri Megadim[10] proposes a further limitation on the basic ruling. Noting that the case discussed by the Rishonim involved people who thought that they were supposed to read the Chanuka portion first, he says that if the oleh was aware of the correct reading but was inadvertently shown the wrong column, then he does not make another beracha. Although the classical poskim and the Mishna Berura apparently reject the Pri Megadim and the accepted practice seems to follow the Mishna Berura, the Pri Megadim makes a lot of halachic sense. The Radvaz[11] raises a further point, namely, that the beracha relates primarily to the mitzva of public Torah reading, with the specific text being secondary. Of great importance is the fact that leading recent poskim,[12] including Rav Moshe Feinstein,[13] accept the position of the Pri Megadim and the fact that we try to avoid questionable berachot.
We suggest the following (if the rav is not present). If you recall that the shul’s practice is to follow the Mishna Berura and that the correct column was not visible to the oleh when he made his beracha, have him make a new beracha, unless he is Sephardic, he refuses, or you expect him to be upset if told to repeat the beracha. Otherwise, do not instruct the oleh to make a questionable beracha, given the opposition of many important poskim.