1) THE WRITING ON THE TABLETS
OPINIONS: Rav Chisda teaches that the writing on the Luchos could be read "from the inside and from the outside." What does this mean?
(a) RASHI here (DH v'Nikra) explains that the engraved writing on the Luchos went straight through the stone, from one side to the other.
(b) RASHI in the Chumash (Shemos 32:15) appears to explain differently. The letters not only went all the way through the Luchos, but they miraculously were in the proper, readable direction on both sides. (The MAHARAL in GUR ARYEH, however, asserts that Rashi in Shemos means to say the same explanation that he says here in the Gemara.)
(c) According to the Yerushalmi in Shekalim, not only were the letters reversed on the second side (so that they were in the proper, readable direction), but the rest of the word was also miraculously reversed, so that the entire text could be read from either side of the Luchos. (See also SIFSEI CHACHAMIM to Rashi, Shemos 32:15.)
2) EXAMPLES OF WORDS IN REVERSE
QUESTION: When the Gemara explains how the letters could be read from both sides of the Luchos, the Gemara gives three examples of words with their reversed readings -- "Nevuv," "Behar" and "Saru." Why does the Gemara choose these three words over any other words? These words were not even in the Luchos! (TOSFOS DH Nevuv)
(a) Perhaps the Gemara's intention is to prove its previous statement, in which it was stated that the letters were carved into the Luchos such that they penetrated to the other side.
When the Jews sinned with the Golden Calf, Hash-m said to Moshe, "Saru Maher Min ha'Derech" -- "They have gone wayward quickly away from the path that I have commanded them..." (Shemos 32:8). The Chachamim relate that when the Jews sinned with the Golden Calf, the letters on the Luchos "flew to the heavens" and no trace of them remained on the Luchos. How did that happen? The letters had no "backs" -- that is, there was no surface at the back of the engraved letter, since it penetrated straight through the Luchos. When the Luchos fell apart, no part of the letters remained -- since they were comprised of hollows in the Luchos rather than actual letters engraved on the Luchos.
This is alluded to by the words the Gemara here chooses. The word "Nevuv" appears in Shemos 27:8 and means "hollow." When the Gemara uses this word as an example of a word and its reverse, it alludes to the Luchos that Moshe Rabeinu was given at Har Sinai that were "hollowed out" (that is, the writing punctured the stone from one side to the other). The hollowed-out letters flew off the Luchos (and left an empty impression, or a "Buvan," the reverse of "Nevuv," from the word "shadow") when the Jews sinned "Behar," while they were still at the mountain (as the Gemara says in Shabbos 88b). (They sinned because their arrogance -- their "Rahav" led them to sin.) The letters then left ("Saru") the Luchos (and the Luchos fell apart into small pieces, "v'Ras," from "Resisin," since every letter that was carved into the Luchos was broken into pieces). (Based on Hagahos Ya'avetz)
(b) The BEIS HA'LEVI (Derashos, Derush 18) explains that in the first set of Luchos, the entire Torah was written upon the Luchos miraculously. These three words also appeared on the Luchos, since they appear in the Torah. (See Rabbi Mordecai Kornfeld's "Torah from the Internet," Parshas Ki Tisa.)
3) A "CHES" THAT TURNED INTO TWO "ZAYINS"
QUESTION: The Mishnah states that if one intended to write the letter Ches and instead wrote two Zayins, he is exempt. The Gemara (beginning of 105a) explains that he is exempt because he did not write the three Tagin on top of each Zayin, and thus his Zayins are not complete letters. It seems from the Gemara that if he did write the Tagin, he would be Chayav.
Why would he be Chayav? The Gemara earlier (73a) says that if one intended to throw an object a distance of two Amos in Reshus ha'Rabim but instead threw four Amos, he is exempt. Here, too, he should be exempt even if he writes the letters with the Tagin, because he intended to do something that was not forbidden (write a single Ches), which is equivalent to throwing a distance of two Amos!
ANSWER: TOSFOS (DH Niskaven) explains that the Mishnah refers to a case in which one intended to write two letters, the first of which was meant to be a Ches. Since he intended to write two letters, he is Chayav when he writes the two Zayins properly.
4) HOW TO WRITE A "CHES"
OPINIONS: The Mishnah says that it is possible for a person to intend to write a Ches and to end up writing two Zayins instead. The Gemara explains that the Zayins must have "Tagin," three small "crowns," in order for one to be Chayav for writing them, since a Zayin is written with Tagin in a Sefer Torah.
How is a Ches written?
(a) In K'SAV BEIS YOSEF, a Ches is comprised of two Zayins with an upwards, pointed angle that attaches them. The Mishnah here that discusses one who intended to write a Ches but accidentally wrote two Zayins seems to be a proof for this opinion.
(b) According to the ARIZAL, a Ches is comprised of one Zayin and one Vav, with an upwards, pointed angle that attaches them. If one skips the angle, a Vav and a Zayin should remain. According to this opinion, why does the Gemara here call the Vav a Zayin and insist that one must add Tagin to it in order to be Chayav?
(c) In KESAV VELLISH, the Ches looks like an ordinary Ches that is in printed in Sefarim. This seems to be the opinion of RASHI in Menachos (29b). RASHI here (DH Niskaven) also follows this view, when he implies that the person did not write any part of the roof of the Ches. Why, then, does the Mishnah discuss one who writes two Zayins, and not two Vavs?
Perhaps when one intends to write a Ches, which is a letter of significance, in order to be Chayav he must write two other letters of significance. This excludes Vavs and Yuds, which are not considered significant because they are only a scratch and a dot. For this reason, the Mishnah says that two legs of the Ches must have crowns on top, to make them Zayins, because otherwise one has not written letters of significance. This approach also explains why the Mishnah says two Zayins (and not two Vavs) according to the Arizal (for one will be Chayav only for writing two Zayins, but not two Vavs).