Inspired by this comment, and thinking about how we today look back on (for example) segregation, or the treatment of homosexuals through the last 100 years.
In the year 2120 what do you think human society will look back on and be disgusted to think about?

The big one for me I think will be the treatment of animals, and not just battery farming, but straight up growing them for slaughter: food, clothes, lab-testing, etc. With the nascent industry of 3d-printed/lab-grown meats gaining traction, as well as vegan and vegetarianism on the rise through much of the West, it's not hard to imagine our great-grandchildren being horrified at the thought of their ancestors raising animals just to kill them.