Apopulation expert makes a graph to show the future rates of population growth in various countries. which is necessary a source of error for him?

The error of the expert is the fact that future trends cannot accurately be predicted.
Population growth is difficult to predict because we don't know the future events that can alter birth rates, death rates, migration, or resource limitations.
For example a disease starts to kill a lot of people, affecting the growth of the population

The solid-liquid line in the water phase diagram has a negative slope, whereas for most other substances it has a positive slope.