What are the most common causes of a 'fatal error sans theme failed creation' in WordPress?

Common causes include: insufficient PHP memory limit, an outdated or incompatible PHP version, incorrect file permissions for theme folders/files, corrupted theme files during upload, conflicts with existing plugins or the WordPress core, database connection issues, or syntax errors within the theme's core files (e.g., `functions.php`). Sometimes, a poorly coded theme or a theme that relies on specific server configurations not present can also trigger this.