Chukat - Shlishi
"Listen up, you rebels!"
G-d told Moshe to take his staff and assemble the people by a certain rock. There, he was to speak to the rock in the people's presence and it would give forth water. Moshe took his staff and gathered the people, but then he went off-script.
"Listen up, you rebels!" he shouted. "Let's see if we can get water for you from this rock!" (Usually I point out that direct quotes are pretty loosely translated, but not here. That's pretty much exactly what he said.) Moshe struck the rock with the staff - twice - and water came gushing out.
G-d told Moshe and Aharon that they showed a lack of faith and failed to sanctify Him. As a result, they would not be bringing the nation into the land of Israel. The place where this incident occurred became known as Mei Meriva, "Waters of Dispute."
The question remains as to what precisely Moshe did wrong. The most obvious answer, cited by Rashi, is that he struck the rock, rather than speaking to it as instructed. (You may recall the Moshe was instructed to strike a rock under similar circumstances, way back in parshas Beshalach, Exodus chapter 17.) Another explanation is the Moshe lost his temper with the people, screaming "Listen up, you rebels!" when lack of water in the desert was a reasonable complaint. There are many other opinions. What they all have is common is that they seem like minor offenses. A person like Moshe, who was so close to G-d, was judged by a more exacting standard than most people, since people would scrutinize his every action. To utilize a metaphor, the cleaner the garment, the more obvious a tiny stain would be.