404 error
A 404 error is the response a server returns when a requested page doesn't exist. A few are normal on any site; a sudden increase after a redesign is usually the sign that URLs changed without redirects.
In practice
The code means "not found" rather than "broken." It's the correct answer when a page genuinely never existed, and it's the wrong answer when a page moved and nobody mapped it. That distinction is the whole issue: search engines and buyers treat a 404 as a dead end, so serving one for content that still exists somewhere is a self-inflicted loss.
The expensive version is the post-launch spike. A rebuild ships, the top pages are redirected, and a few hundred deeper URLs quietly start returning 404s. Because those pages were individually small, nobody notices for weeks. Collectively they were often a meaningful share of qualified traffic, since a deep page tends to be the specific one a technical buyer searched for.
There's also a version that's worse because it hides: the soft 404, where the server reports a page as working but the content says nothing is there, or every missing URL gets redirected to the homepage. Search engines increasingly identify these and treat them as not found anyway, so the workaround provides no benefit while making the problem invisible in your own reporting.
When a page is genuinely gone with no equivalent, a 404 is the honest response and you should leave it. Not every removed page deserves a redirect. Redirect where there's a real destination the visitor wanted; return a 404 where there isn't, rather than sending them somewhere irrelevant.
The 404 page itself is worth ten minutes of attention, because the people seeing it arrived with intent. Search, main navigation, and a link to your primary services page recover a visit that would otherwise end. A page saying only "Page not found" throws away someone who was actively looking for you.
Common questions
Do 404 errors hurt SEO?
A few are normal and harmless. What causes damage is a large number appearing after a redesign, because that means URLs with existing rankings and inbound links now lead nowhere.
Should I redirect every 404 to the homepage?
No. Search engines commonly treat a bulk redirect to the homepage as a soft 404, so it passes little value, and it strands the visitor. Redirect to the closest equivalent page, or return a genuine 404 where no equivalent exists.
What is a soft 404?
A page that reports success to the server but has no real content, or a missing URL redirected somewhere irrelevant. Search engines identify these and treat them as not found, so the workaround gains nothing and hides the problem.