The golden calf EXO 32:4

After Moses had been on Mount Sinai for forty days, the people decided he was not coming back and asked Aaron to make them a god. Aaron collected their gold earrings and cast them into a calf, and the people declared it the god who had brought them out of Egypt. A feast was held the next morning, and when Moses came down from the mountain he found the camp singing and dancing around it.

The whole episode happens at Mount Sinai, at the foot of the very mountain where Moses was receiving the tablets of the covenant. It is at that same base of the mountain that he comes down and shatters the tablets once he sees what the people have done.

It matters because the covenant that Israel had just entered with God is broken within weeks of being given, and Moses marks that breach physically by smashing the tablets at the foot of the mountain. The timing is what makes it so stark: the people turn to an idol while the terms of their relationship with God are still being written down above them.

It is easy to assume the people were simply swapping the Lord for a foreign god, but they call the calf the god who brought them out of Egypt, the very language used elsewhere for the Lord himself. That has led Christians to read the offense differently, some seeing a rejection of God outright and others an attempt to picture him in a forbidden image. Either reading agrees that the result was the same: a covenant broken almost as soon as it was made.