Explosion of CSS selectors
The blind adoption of the BEM methodology combined with a (wrong) use of Sass @extends took to these insane outcomes, where the resulting CSS selectors were simply un-manageable and un-debuggable. Not only: the size of the generated CSS file was around 1.2MB! After one year of hard refactoring, using a modular and "atomic" approach, I've been able to reduce the size of the CSS files to around ~100KB max each (and the CSS selectors are back to being human-readable).