Thank you for the Biblical question on the fateful occasions when the Israelites complained to Moses, “What are the three fateful occasions when the Israelites complained to Moses?”
As I read over the Book of Numbers, the clear fateful occasions are as follows:
1. Numbers 14:1–45 on the rebellion to enter the Promised Land whereby that generation died in the wilderness.
“That night all the members of the community raised their voices and wept aloud. 2 All the Israelites grumbled against Moses and Aaron, and the whole assembly said to them, “If only we had died in Egypt! Or in this wilderness! 3 Why is the Lord bringing us to this land only to let us fall by the sword? Our wives and children will be taken as plunder. Wouldn’t it be better for us to go back to Egypt?” 4 And they said to each other, “We should choose a leader and go back to Egypt.”
26 The LORD said to Moses and Aaron: 27 “How long will this wicked community grumble against me? I have heard the complaints of these grumbling Israelites. 28 So tell them, ‘As surely as I live, declares the LORD, I will do to you the very thing I heard you say: 29 In this wilderness your bodies will fall—every one of you twenty years old or more who was counted in the census and who has grumbled against me. 30 Not one of you will enter the land I swore with uplifted hand to make your home, except Caleb son of Jephunneh and Joshua son of Nun. 31 As for your children that you said would be taken as plunder, I will bring them in to enjoy the land you have rejected. 32 But as for you, your bodies will fall in this wilderness. 33 Your children will be shepherds here for forty years, suffering for your unfaithfulness, until the last of your bodies lies in the wilderness. 34 For forty years—one year for each of the forty days you explored the land—you will suffer for your sins and know what it is like to have me against you.’ 35 I, the LORD, have spoken, and I will surely do these things to this whole wicked community, which has banded together against me. They will meet their end in this wilderness; here they will die” (A searchable online Bible in over 150 versions and 50 languages). Bible citations are from the New International Version.
2. Numbers16:42–50 on the grumbling of the death of the censers by the people whereby a plague broke out of 14,700 died because of Korah.
But on the next day all the congregation of the sons of Israel grumbled against Moses and Aaron, saying, “You are the ones who have caused the death of the LORD’s people.” 42 It came about, however, when the congregation had assembled against Moses and Aaron, that they turned toward the tent of meeting, and behold, the cloud covered it and the glory of the LORD appeared. 43 Then Moses and Aaron came to the front of the tent of meeting, 44 and the Lord spoke to Moses, saying, 45 “[ad]Get away from among this congregation, that I may consume them instantly.” Then they fell on their faces. 46 Moses said to Aaron, “Take your censer and put in it fire from the altar, and lay incense on it; then bring it quickly to the congregation and make atonement for them, for wrath has gone forth from the LORD, the plague has begun!” 47 Then Aaron took it as Moses had spoken, and ran into the midst of the assembly, for behold, the plague had begun among the people. So he put on the incense and made atonement for the people. 48 He took his stand between the dead and the living, so that the plague was checked. 49 But those who died by the plague were 14,700, besides those who died on account of Korah. 50 Then Aaron returned to Moses at the doorway of the tent of meeting, for the plague had been checked” (NIV).
3. Numbers 21:4–9 on the grumbling of the people against Moses in having 250 people whereby snakes killed an unknown number of people.
“4 Then they set out from Mount Hor by the way of the [f]Red Sea, to go around the land of Edom; and the [g]people became impatient because of the journey. 5 The people spoke against God and Moses, “Why have you brought us up out of Egypt to die in the wilderness? For there is no [h]food and no water, and [i]we loathe this miserable food.”
6 The LORD sent fiery serpents among the people and they bit the people, so that many people of Israel died. 7 So the people came to Moses and said, “We have sinned, because we have spoken against the LORD and you; intercede with the LORD, that He may remove the serpents from us.” And Moses interceded for the people. 8 Then the LORD said to Moses, “[j]Make a fiery serpent, and set it on a standard; and it shall come about, that everyone who is bitten, when he looks at it, he will live.” 9 And Moses made a bronze serpent and set it on the standard; and it came about, that if a serpent bit any man, when he looked to the bronze serpent, he lived” (NIV).
There are other passages that implies that the people complained to Moses, but it doesn’t state it directly with the resulting death that happened to the people.
I hope this answers your question and I will be opened to what you observe from the text.
For more perspectives:
https://www.quora.com/What-are-the-three-fateful-occasions-when-the-Israelites-complained-to-Moses