You know what really grinds my gears? Code duplication. And you know where you can find plenty of that? In WordPress themes! This has gotten a little better recently, thanks to get_template_part(), but it’s still not a complete solution.
Ever since I’ve taken over development of the WP-PageNavi plugin, I keep seeing people opening support topics like “PageNavi doesn’t show the correct posts” etc.
Most of the time, your theme is using query_posts() wrong.
If you’re a theme developer, you’re probably familiar with the category.php and category-{id}.php template files. The later lets you display a single category differently.
The WordPress theme system allows unlimited freedom to developers, which often leads to suboptimal results and bad practices. This is one of the reasons why several theme frameworks have emerged over time: