Poor commenting
Why the founder of Rails automatically rejects 80% of Software Engineer applicants
Christophe Limpalair

When I’m hiring, I find most people don’t understand the purpose of commenting. As in the example in the article, most people comment on the “what" and not the “why". Code that is self-documenting rarely needs an explanation of what it is doing. And with enough coffee, you can usually figure it out. Once you get to that point, you may discover that you don’t know why it’s doing it. Providing that context is what a good comment does.