Generally, a person who ate a Ke’zayit (the amount requiring one to recite the Beracha after eating) of Mezonot (grain products other than bread) recites the Beracha of "Al Ha’michya Ve’al Ha’kalkala," and one who ate a Ke’zayit of one of the seven special fruits (such as olives, dates and grapes) recites the Beracha of "Al Ha’etz." If a person ate both a Ke’zayit of grain products and a Ke’zayit of one of the seven fruits, he combines them into a single Beracha: "Al Ha’michya Ve’al Ha’kalkala Ve’al Ha’etz Ve’al Peri Ha’etz."
The question was raised concerning a case where one ate a Ke’zayit of grain products, but cannot ascertain whether the fruit he consumed amounts to a Ke’zayit, and he is therefore unsure whether or not he must include "Ve’al Ha’etz" in his Beracha . Since he must in any event recite "Al Ha’michya," should he mention as well "Ve’al Ha’etz Ve’al Peri Ha’etz" to cover the possibility that he in fact did eat a Ke’zayit of fruit? Or, do we say that according to the possibility that he need not include "Ve’al Ha’etz," mentioning it in his Beracha will constitute a Hefsek – an unlawful interruption in the middle of the Beracha?
Maran (author of the Shulchan Aruch) explicitly addresses this question (in the Orach Chayim section, Siman 208), and writes as follows: "One should not include out of doubt any addition in the Beracha Me’ein Shalosh, even though he does not add Shem U’malchut [the phrase ‘Hashem Elokenu Melech Ha’olam’]." Thus, if one is uncertain whether the addition belongs in the Beracha, he should not include it. In our case, then, the person should recite only "Al Ha’michya Ve’al Ha’kalkala," without adding "Ve’al Ha’etz Ve’al Peri Ha’etz."
The Taz (classic commentary on Shulchan Aruch by Rabbenu David Halevi, 16th-17th century, Poland), however, advances a very novel and interesting reading of this Halacha in the Shulchan Aruch. He understands the Shulchan Aruch as requiring one to avoid this situation, where he is unsure whether or not he must include "Ve’al Ha’etz Ve’al Peri Ha’etz." After the fact, however, if this did occur, then the individual must, according to the Taz, add "Ve’al Ha’etz" given the possibility that he did eat a full Ke’zayit of fruit. The Shulchan Aruch, he claims, meant only that optimally one should not put himself in this problematic situation.
However, this position is held only by the Taz. Hacham Ovadia Yoseph and most Acharonim (authorities after the Shulchan Aruch) disagree, and indeed the straightforward reading of the Shulchan Aruch seems to support their position. Therefore, if a person finds himself in such a predicament, where he must recite "Al Ha’michya" but is unsure whether he must mention as well "Al Ha’etz," he does not include "Al Ha’etz," given the uncertainty as to whether or not it belongs in the Beracha.