Summary: It does not take much time or much trouble to get some folks to grumble. We certainly see this with the Israelites of old, and we can see it today.
1 When the people complained it displeased the LORD. When the LORD heard it, his anger burned, and so the fire of the LORD burned among them, and consumed some of the outer parts of the camp. 2 Then the people cried to Moses; and Moses prayed to the LORD, and the fire died out. 3 And he called the name of that place Taberah, because the fire of the LORD burned among them (Numbers 11:1-3).
After only three days, the people are already complaining against God, and the reasons seem to be so petty that they are not even mentioned (as they are elsewhere). I cannot help but think that the mention of being three days into their journey (10:33) was very deliberate. I suspect Moses is trying to cause the us to remember that Israel began to grumble just three days after they had crossed the Red Sea:
22 Then Moses caused Israel to journey from Yam Suph, and they went out into the desert of Shur. They went three days into the desert, and they found no water. 23 Then they came to Marah, but they were not able to drink the waters of Marah, because they were bitter. (That is why its name was called Marah.) 24 So the people murmured against Moses, saying, “What can we drink?” (Exodus 15:22-24)
It does not take much time or much trouble to get some folks to grumble. We certainly see this with the Israelites of old, and we can see it today. The emphasis in verses 1-3 is not so much on the displeasure of the Israelites as it is on the displeasure of God with the Israelites. God was angry because His people complained. God responds in a manner that I would liken to “firing a shot over the bow.” The “fire of the Lord” (lightning?) came down from heaven, consuming some of the outer portions of the camp. It is difficult to determine whether or not any people were destroyed. Since this “fire” struck the outer portions of the camp, it may have been that some of the Israelites’ animals were consumed. The warning should have been very clear. God was greatly displeased with their grumbling, and He would not tolerate it.
One would expect that God’s response to Israel’s grumbling in verses 1-3 would have silenced any future protest, but this was hardly the case. The “mixed multitude” (some translations say “rabble”) who accompanied the Israelites when they left Egypt (Exodus 12:38) began to complain. Was it due to some very unpleasant or dangerous circumstance? Hardly. These folks complained that their food wasn’t as spicy as the food they had eaten in Egypt:
4 Now the mixed multitude who were among them craved more desirable foods, and so the Israelites wept again, and said, “If only we had meat to eat! 5 We remember the fish we used to eat freely in Egypt, the cucumbers, the melons, the leeks, the onions, and the garlic. 6 But now we are dried up, and there is nothing at all before us except the manna.” 7 Now the manna was like coriander seed, and its color like the color of bdellium. 8 And the people went about, and gathered it, and ground it with mills, or pounded it in mortars; they baked it in pans, and made cakes of it. Its taste was like the taste of fresh olive oil. 9 And when the dew came down on the camp in the night, the manna fell with it (Numbers 11:4-9).