The gemara proves that it is vital for a Rebbi to be medakdek when teaching students from the fact that the Rebbi of Yoav wasn't careful when teaching תמחה את זכר עמלק and misled yoav to read that the males of amalek must be destroyed, but not the females. The gemara says that when they asked the rebbi how he taught, the rebbi responded זכר, and they wanted to kill him. Tosafos explains that had the rebbi himself made a mistake by pronouncing it "zachar" with a kometz, he wouldn't have deserved to be killed. But since the rebbi claimed to have taught "zeicher", it means he was careless in being medakdek with his talmidim, so he deserved to be killed.
Alternatively, the maharsha says that really the mistake was made at the level of the rebbi of yoav who actually thought that it should be read "zachar" with a komatz. Why was he punished so severly for not knowing? The maharsha explains:
דלא הוה ליה לקבל על עצמו להיות מקרי דרדקי ומקרי מלאכת ה' רמיה שהרי הוא היה יודע בעצמו דלא הוה גמיר שפיר
A Rebbi must know before accepting to teach children that he is proficient and can teach flawlessly. For accepting a teaching job that he wasn't proficient to do, he was chayev misah.
The Poras Yosef offers a peshat in this gemara from where he proves that the correct reading is zeicher with a tzeirei, rather than zecher with a segol. In dikduk the vowel of tzerei is also called a "komatz kattan". The rebbi of yoav didn't pronounce the word, rather he explained that it should be read with a "komatz". The rebbi was careless in pointing out that it was a komatz kattan so that it should be pronounced "zeicher", not a real komatz which is pronounced "zachar" which led to the mistake of yoav. This proves that the correct reading is with a tzeirei, which is komatz kattan, not a segol. How? Because if the correct reading was with a segol the mistake could not have happened since a segol has no similarity with komatz to lead to a mistake. According to this approach the proper pronunciation is a tzeirei, but maharsha quotes rashba that it should be read with a segol.