The Problem register_post_type() called in plugins_loaded — and the site crashes with a fatal error, because taxonomies aren't ready yet. Or add_theme_support( 'post-thumbnails' ) called in init — no error, thumbnails just quietly never show up, because WordPress has already passed the point where a theme declares feature support. Or a plugin's text domain gets loaded after translatable strings have already been output, and the translation simply doesn't take. // Too early: ...