Toward the end of the parsha the Torah describes Bnei Yisrael's dire situation with regard to the lack of water in the wilderness. "There the nation thirsted for water and they complained to Moshe, 'Why have you brought us out of Mitzrayim - to kill me, my children and my animals through thirst?'" (Shemos 17, 3). Consequently, Moshe turned to Hashem for help: "What should I do for this nation? A little bit more and they will stone me!" Hashem responded, "Pass in front of the nation" with your staff in hand and hit the rock and water will pour forth. Rashi explains that Hashem responded, "Why have you falsely accused My children?" He then told Moshe specifically to "pass in front of the nation" to prove that Bnei Yisrael would not stone him.
Rav Wolbe (Shiurei Chumash) comments that Moshe certainly was not exaggerating. If he stated that he was afraid that Bnei Yisrael were going to stone him, then he must have felt that Bnei Yisrael were arguing with him so vehemently that they had reached a point where killing their leader was only a matter of time. Nevertheless, Hashem rebuked Moshe, "Why have you falsely accused My children."
In a similar vein, the Medrash (Mechilta) says that there were three prophets: one defended the honor of the father while neglecting the honor of the son, one defended the honor of the son while neglected the honor of the father and the third defended the honor of both the father and son. Eliyahu defended the honor of Hashem (the Father) when he stated, "I have been zealous for the sake of Hashem." Yonah defended the honor of Bnei Yisrael (the children) when he ran away to avoid delivering the prophecy to the city of Ninveh lest their teshuva act as an indictment against Bnei Yisrael who failed to do teshuva. As a result, both of them were punished and Hashem terminated their role as neviim to Klal Yisrael. In contrast, Yirmiyahu, defended both Hashem and Bnei Yisrael when he declared, "We have sinned and rebelled, and You have not forgiven us." He was duly rewarded and his nevuah was doubled. Once again we see how careful one must be when speaking not only about Hashem Himself, but about his children as well.
If neviim were taken to task about the way they spoke about Bnei Yisrael, how careful must we be when referring to any of our brethren? They are not only our brothers; they are the children of the Creator. Even a derogatory word about a single Jew is inexcusable, how much more so when the subject of negativity is an entire group in Klal Yisrael! Words are powerful, and if they are used properly they can garner rewards on the caliber of doubling a prophet's prophecies!