Why it matters
When a page doesn't exist your server still answers 'all good' (a 200). Search engines then index junk URLs, and scanners can't tell real files from fake ones.
How to check it yourself
Visit a made-up address on your site and check the status code in DevTools' Network tab. 200 for a page that doesn't exist is a soft 404.
What the fix involves
Return a real 404 status with a friendly not-found page. Single-page apps need the host configured so only real routes fall back to index.html.
How vibeliq checks it
We load your page the way a visitor's browser would, read what your server sends back, and look at a short list of well-known addresses on your site. If this turns up, your report explains it in plain English and gives you a prompt, written for Claude Code, Cursor, Lovable or whatever built your site, that fixes it in your actual codebase.