The warning appears when PHP receives a variable that is not iterable in a foreach loop. WordPress core functions, plugins, themes, or custom code often pass null, a string, or an undefined variable where an array or object is expected. This situation triggers the PHP runtime to emit the Invalid argument supplied for foreach() notice.
Typical triggers include uninitialized global objects such as $wp_styles->queue, outdated core files that assume a property is always an array, plugins that deregister core objects, and options stored in the database that return null instead of an array. When the loop runs without a valid iterable, PHP stops normal output and might cause additional header errors.