Why did wearing a Sadin cause Rav Katina not to fulfill the Mitzvah of Techeles?

Rashi: We said above (40b) that Chachamim decreed not to put Tzitzis on a linen garment. This was a decree due to a night garment (there is no Mitzvah to override the Isur of Kil'ayim. Here, 'Techeles' means Tzitzis.)

'Rashba': It had Lavan, but not Techeles.

Why did wearing a mantle cause Rav Katina not to fulfill the Mitzvah of Techeles?

Rashi: It is round, and has only two corners, below. `

Why did Rav Katina think that there is no punishment for not fulfilling a Mitzvas Ase?

Tosfos: Tzitzis is more lenient. One is not obligated to buy a garment obligated in Tzitzis. An absolute Ase, even a human Beis Din lashes until the person [fulfills it or] dies!

Maharsha: Bava Kama 55b proves that Shamayim's judgment is harsher. It lists several matters for which one is liable bi'Ydei Shamayim, but not bi'Ydei Adam.

Daf Al ha'Daf: Klei Chemdah (Vayera) infers that the Ritva (Rosh Hashanah 16b) argues with Tosfos. He says that Beis Din above would punish for Bitul Teki'as Shofar only at a time of anger. Ohr ha'Chayim (Shemos 20:1) says that it says Elokim (Shem Din) in Aseres ha'Dibros, to teach that even though normally, Hash-m punishes for Bitul Ase only at a time of anger, for an Aseh in Aseres ha'Dibros, He always punishes.

Daf Al ha'Daf citing Pardes Yosef Chadash (Shlach): Pesach Einayim (Shabbos 25b) points out that R. Yehudah wrapped himself with Tzitzis and resembled an angel. He questions Maharsha, who learns from our Gemara [that angels wear Tzitzis, for we see] that they are adamant about Tzitzis.

Maharal: One who transgresses a Lav does what is improper to do. One who nullifies an Ase, he did not acquire a level that he could have, and will not receive its reward in the world to come. He should not be punished!

Anaf Yosef citing Be'er Mayim Chayim: It suffices that he will not receive the reward, which is infinite. This is why 'there is not reward in this world for a Mitzvah' (Chulin 142) - this entire world does not suffice for the reward for one Mitzvah. Nebuchadnetzar took three steps for Hash-m's honor, and received kingship over the entire world [for three generations]. What will be given to those who toil in Mitzvos with body, Nefesh and money, e.g. Sukah, Esrog and Lulav, or going to Beis ha'Keneses, which requires more than 200 steps! If one is foolish and does not fulfill, he lost reward of seeing the Shechinah - nothing can compare to that pleasure! What greater punishment is there in the world?!

Why do they punish for an Ase at a time of anger?

Maharal: When total Midas ha'Din rules in the world, he is punished, for he should have acquired the attribute. Man was created to constantly ascend levels!

Maharsha: At a time of anger, a Tzadik should protect his generation via Tefilah and Mitzvos. He should increase Mitzvos, and not find strategies to exempt himself! A Tzadik like Rav Katina is punished for this.

Iyun Yakov: Perhaps Moshe was punished for not circumcising his son, for it was a time of anger. Hash-m was angry that Moshe refused the Shelichus [to redeem Yisrael] - "va'Yichar Af Hash-m b'Moshe"! Also, the Satan prosecutes at a time of danger, on the road in the lodging place. Therefore, Af and Chemah sought to swallow him. Tosfos said that for an absolute Ase, even a human Beis Din lashes. Moshe did not transgress, just he engaged in the lodging first (Nedarim 31).

Daf Al ha'Daf citing Pardes Yosef Chadash (Va'eschanan): Moshe said "va'Shem His'anaf Bi Al Divreichem." He was punished for his sin at Mei Merivah! Rather, Yisrael's words (they complained about water - Targum Yonason) caused it to be a time of anger, therefore Hash-m punished Moshe for not speaking to the rock.

Anaf Yosef citing Be'er Mayim Chayim: When Din overpowers, there is Din even for one who [normally], due to Midas ha'Rachamim, would not be liable.

Chashukei Chemed: The Magen Avraham (13:8) said that one is not obligated to toil for a Mitzvah more than the normal amount. If he can do it only with extra toil, he is exempt. Bi'ur Halachah (656) says that this is like Ones. It seems that a time of anger is different. Then, one must toil more for Mitzvos, e.g. go to another city if he cannot find Tzitzis in his city. Certainly he will be rewarded greatly for this, even if he was not obligated.

Daf Al ha'Daf citing Binah l'Itim: He asked 'do you punish...', and it answered 'we punish...' Punishment via an angel is harsher than via Hash-m Himself. David cursed his pursuers "u'Mal'ach Hash-m Rodefam." It says "u'Mal'ach Achzari Yeshulach Bo." Rav Katina understood that there is punishment for refraining from an Ase. He thought that it is not to severe that angels would punish for it. Pnei Yehoshua (Shabbos 55a) asked, the angels that saved Lot were angels of mercy. They left Avraham at midday, and delayed six hours before coming to Sedom, and the entire night before destroying. Why does it say "v'Ya'itzu" (they rushed) Lot? Lot was not worthy to be punished for not protesting against the city. It is only an Ase. One is punished for an Aseh only at a time of anger. Sunrise is the time of anger, for then kings of the east and west bow to the sun (Berachos 7a), and all the more so in Sedom, where people worshipped something sun. (NOTE: There, Rav Elyashiv brought from Sha'agas Arye that Stam kings rise at three hours, but those who worship the sun rise early, in order to bow when it rises. - PF) Therefore, after dawn, which is close to sunrise, they rushed him to leave, lest he perish in the sin of the city at the time of anger. He reached Tzo'ar at sunrise. Teivas Gome asked, Tosfos said that only for an optional Ase, one is punished only at a time of anger! Klei Chemdah (Vayera) answered before Matan Torah, it was not obligatory. There was no Mitzvah of Tochachah! Pnei Yehoshua wrote b'Derech Drush.