The 20th century saw a number of failures that were potentially avoidable, and that all seem to be converging on the mid-late 21st century.
Aside from the many, many failures in the realms of war and economics, there are three big ones that worry me right now, and that I think will feed into each other in a horrific manner. The first, obviously, is climate change. The other two are overpopulation and drug resistance in diseases.