A 404
Your 404 page is more than an error—it's a critical recovery tool. Use 301 redirects to preserve SEO for old URLs, but design your actual 404 page to guide lost shoppers with a prominent search bar, links to bestsellers, and a clear, on-brand voice.
A 404

Rand Fishkin discusses common SEO mistakes, and mishandling 404s is one of the classics to avoid.