404リンクを訪問すると、ヘッダーから機能（アバター、検索、ハンバーガーなど）が剥がされます

[Stephen]

AFAIK this is still the case:

Plugin not loaded when route not found

The error page is a plain HTML page, we don’t load the Javascript app there, so no Ember, JQuery, plugins, etc.

So an error raised by a bad inbound link will serve the flat page, whereas an error from a bad internally navigated link would still contain those elements.