What event changes the tone of Cub Pilot on the Mississippi?

The event that changes the tone of "Cub Pilot on the Mississippi" is when the protagonist, Twain, accidentally steers the steamboat into a wrecked ship in dense fog. This event marks a shift in the narrative from the excitement and adventure of learning to pilot a steamboat to the realization of the dangers and responsibilities that come with the job. Twain's mistake serves as a turning point in the story, highlighting the harsh realities of life on the river and the importance of experience and skill in navigating its treacherous waters.