When you want to do something simple but you have to bring the entire philosophy of CSS into consideration to figure out what the right way to do it is.
Its a common misconception that CSS is easy, that's its not a programming language. Its however one of the most difficult to understand. You can write CSS a whole career without understanding the basics, and responsive design made it exponential harder. And there are popular anti-patterns such as trying to overwrite an existing css file... But if you do learn it - its very powerful, and much better then defining design rules in JS or XML or a graphical design tool.