Scientists develop genetic engineering, cybernetic enhancements, and artificial intelligence to an extremely high level. So much so that for the first time in human history, humanities biological destiny is now able to be completely dictated by the will of the biological creatures in question, not nature. No longer will humans have to deal with diseases, deformities, and all kinds of biological inequality. These anhancements, of course, will also entail the ability to  always think perfectly rationally, free of bias, and large emotional influence. But then it happens.

Human beings become way too similar, too generic. Over the years, people will stop developing both socially and civilly due to the societal changes brought on by the complete homogeneity of the species. Humans will eventually stop thinking like humans and more like ultra efficient machines. All of humanity would then be only concerned with three things, manufacturing, expansion, and forceful assimilation. You have created the Borg. 

I wish that people would stop being pedants in order to seem smart and actually commit intelligent acts.