The sentence with the unclear pronoun is:
"If you set the plant on the shelf over the fireplace, it might fall off the wall and break."
In this case, "it" is unclear because it's not clear whether it refers to the plant or the shelf. To correct it, you could specify which noun "it" refers to. For example, "If you set the plant on the shelf over the fireplace, the plant might fall off the shelf and break."