What happens if a researcher doesn't fulfill the aims put forth in their proposal? (NIH)

I am a senior undergraduate student. In class we have to write a mock grant proposal. I was assigned NIH.

While writing it, I began to think about what would happen if a researcher couldn't complete all of the aims before the grant period ends? It seems like while writing the grant, it may be easy to over shoot a reasonable aim. What happens in that case?