Some classic problems many have faced with CSS have been made really easy and trivially solved by Flexbox. Philip Walton in his solved-by-flexbox project lists 6 classic problems (as of this writing) and extensively discusses the previous limitations with CSS and the current solution flexbox provides.
I advice you go take a look. It's so good, I could have lifted the contents. However, in the practical section coming up, I'll explain some of the concepts he addresses as I walk you through building a practical layout with flexbox.
If you're the one the writes CSS in their dreams and even if you're not, you should watch this github repository. Some people smarter than I am curate a list of flexbox bugs and their workarounds there. It's the first place I look when something isn't working as I expect.
I'll be working you through some prominent bugs in the practical section below too. So you're covered.
Next Read: BUILDING A MUSIC APP LAYOUT WITH FLEXBOX