Any HTTP failure (404, 500, timeout, blocked resource) a search engine hits while trying to fetch a URL. Reported in Google Search Console under Coverage.
Crawl Error in plain English
When Googlebot tries to fetch a URL and fails, the failure type is recorded. The main categories: 4xx errors (page not found, page gone, page forbidden), 5xx errors (server crashed, server overloaded, timeout), redirect issues (chains too deep, redirect loops), and resource-blocked errors (robots.txt disallowed, server rejected the request). Each type tells you something different about what's wrong.
4xx errors are usually content issues — links to URLs that don't exist, deleted pages still in sitemaps, broken internal navigation. 5xx errors are infrastructure issues — server is crashing, database queries are slow, the app is running out of memory. The two require completely different fixes. 5xx errors are particularly dangerous because Google interprets a pattern of 5xx responses as 'this site is unreliable, slow down crawling' — which means your important pages get crawled less often.
Google Search Console's Coverage report (now called Indexing → Pages) groups crawl errors by type and shows trend data. A sudden spike in 5xx errors usually means a deploy broke something or traffic overwhelmed your server. A gradual rise in 404s usually means a CMS or sitemap is out of sync with reality. Either way, the spike is the signal.
The fix flow: triage by status code. For 4xx: are these URLs supposed to exist? If yes, fix the link or restore the page. If no, leave them — Google will deindex. For 5xx: check server logs around the spike time, investigate the underlying infrastructure issue. For redirect chains: audit with Screaming Frog and collapse chains to single hops. For robots.txt blocks: review whether you intended to block those URLs.
Why crawl error matters for ecommerce
Crawl errors are the early-warning system for SEO disasters. A migration that broke URL handles, a CDN misconfiguration that made some pages serve 502s to Googlebot, a sitemap generator that's stale — all of these surface in Search Console's crawl error reports before they cost you traffic. Set up email alerts for new errors and check the report at least weekly. For ecommerce stores especially, crawl errors during peak retail traffic (Black Friday week) directly translate to lost organic revenue.