From Chabad.org: G‑d tells Moshe and Aaron to take a staff and gather the people in front of a certain rock. They should speak to the rock, and it will give forth water. Moshe and Aaron gather everybody, and Moshe strikes the rock and it gives forth water. In the course of this episode they committed a grave error, the conventional explanation being that they struck the rock instead of speaking to it. This caused G‑d to punish Moshe and Aaron, barring them from leading the Jews into Israel.
This is probably the sin that contributed the most to what happened to the Jews. The Egyptian astrologers saw that Moshe will be punished because of something to do with water (this event, where he hit the rock instead of speaking to it), and that’s why they decided to throw all Jewish babies into the water, and that’s how Moshe was saved, etc. There is so much to learn from what Moshe did wrong, why the punishment fit the crime – this Aliya is dripping with lessons (pun intended).