Back in medieval times, serfs had the responsibility of doing hard labor and chores given to them by the lords in return for protection. This was unfair because we did not have any freedom or rights.
Back in medieval times, power lied in the hands of the monarch, also referred to as royalty (kings and queens). The monarchs passed down the throne to members of the family, making it nearly impossible for others to work for power or be voted in. This was unfair because power would never be let out of a certian family line.
Back in medieval times, people were born into being a knight. It was the knight's obligation to fight and protect the land as well as the lords, serfs, and the monarch. This was unfair because they had no choice but to fight in combat when needed.
In the modern era, there are currently children who are not provided with a proper education. They do not have the resources to attend school and are forced into child labor. This is unfair because everyone should have a right to have an education and participate in schooling.
In the modern era, there are currently millions of people living in poverty. High taxes, debt, and different insurances that some agree are ridiculously high. Some people also are unemployed and not earning enough of an income to support everyone in their household or themselves. This is unfair because all some people do is work but they do not make enough money to support people. This could be because they did not have the resources to attend college to get a high income job but get kicked out of their apartments or houses because they cannot afford rent.
It's the 1900s, and women's rights don't exist yet! Can you believe it? Women couldn't do a large fraction of the things men could do because we are known as simply a "the more inferior gender". This was so unfair because, like racism, the discriminated party didn't do anything wrong for these feelings to be had. We are just seen as the weaker class and discriminated against for something we have absolutely no control over.