(a) Rav, one of the primary members of Talmud Bavli, a Godol Hador and presumably a gentle person who loves fellow Jews and especially talmidei chachomim, comes to town with a stomach problem. He is in pain, per the Artscroll, and when things don't go the way he prefers them to, he curses one Rav with a horrible disfigurement that lasts forever, and then he curses another Godol Hador with another horrible curse. His suffering was temporary, but his curses were permanent.
Besides being vindictive and nasty, there was no being don lekaf zchus that maybe Shmuel was a decent chap who was trying to be helpful. And Rav could have avoided everything by simply excusing himself and ask that the questions be asked later, after he went to the bathroom. A simple apology rather than strong curses. I'm sure there is more to the story than the way it seems?
(b) And the curses did not seem to correspond to his pain in any way except to be vindictive. Your name is Karna, so your punishment has nothing to do with what you did, but only with your name. Had his name been Shaul, he might have been sent to the grave (She-ol).
Avrohom Meyer Kohn, Los Angeles, CA USA
(a) The Yavetz explains that there were two problems with Karna's behavior: he delayed Rav from going to the bathroom when Rav was suffering, and additionally did not show the basic courtesy shown to someone who has just arrived from the road, which is the ability to let him recuperate from his journey before engaging him in intense, challenging, Torah study.
The fact that his curses were permanent is not surprising, as people often have to atone for Aveiros for much longer than they engaged in the actual Aveirah. (Take the Meraglim as an example, there are many more). This is because the magnitude of even a fleeting sin can be very severe.
After further research, I found a Chazon Ish which explains this Gemara. He maintains that Rav held that he was the Gadol Hador, and as such should not be tested on Torah (see Rashi DH "Teipuk"), but rather asked questions about Torah. Since he felt that he was obviously being tested, he held that it was a slight to the honor of the Torah, for which Karna deserved a severe punishment.
Once he saw that Karna was Shmuel's friend, and Shmuel gave him a Refuah that did not work and, to the contrary, made things worse, he felt that it was clear that Shmuel was also not a good person who did not respect Kavod Hatorah, and therefore should not have children. Why this curse? The Chazon Ish explains that since the Gemara earlier (23b) stated that someone who honors Torah scholars has children who are Torah scholars, someone who does not, probably will not have good children, and therefore he said that Shmuel should not have children.
(b) Regarding the curse based on a name, it appears that a person's name has to do with his essence, both his good abilities and his bad traits. This is apparent from the many places where Chazal Darshan someone's name (i.e. Korach). In my Sefer Yoshev Ohalim, I suggested the idea that whether these Derashos are for good or for bad, they highlight a positive or negative trait which is inherent from birth (called a Techunah, not to be confused with a Midah) which can be found within the person. It is therefore possible that Rav's curse was based on cursing the negative trait of Karna, which led him to do this sin.
Kol Tuv,
Yaakov Montrose