A 400 Bad Request error is an HTTP status code that tells you the server understood your request but won't process it because something about the request itself is broken. This is a client-side error, meaning the problem originates from the browser, application, or how the URL was constructed, not from the server's ability to handle requests. Common causes include malformed URL syntax (extra characters, incorrect encoding), oversized headers or cookies that exceed server limits, corrupted browser cache forcing bad requests to repeat, invalid file uploads, or improperly formatted API calls with missing required parameters. If you're seeing this on your own site, check for broken JavaScript making malformed AJAX requests or forms submitting corrupted data. From an SEO perspective, 400 errors are less common than 404s but more problematic when they occur systematically. Google can crawl and index pages that throw 400s inconsistently, but if Googlebot consistently receives 400 responses, those URLs get dropped from the index. Check Search Console's Crawl Errors report and server logs to identify patterns. Troubleshooting steps: Clear browser cache and cookies first, since stale data causes many 400s. Test the URL in an incognito window or different browser. If you control the site, check server error logs for specific rejection reasons, review recent code deployments that might have broken request handling, and verify your CDN or firewall rules aren't overly aggressive in rejecting requests. At Ottawa SEO, we see 400s most often after migrations where URL parameters weren't properly mapped, or when clients implement security tools that mistakenly flag legitimate traffic. Unlike 404s which you can redirect away, fixing 400s requires identifying why requests are malformed. If it's affecting one URL, fix that page's code. If it's site-wide, you've got a server configuration or application-level issue that needs immediate developer attention before you lose crawl budget and rankings.