Technical SEO post-migration: How to find and fix hidden errors

Website migrations: A minefield of SEO surprises

Website migrations are notoriously challenging, even for seasoned technical SEOs. No matter how meticulous your planning, unexpected issues inevitably arise. Post-migration monitoring, therefore, is as vital as the migration itself, especially during the crucial first month when hidden problems often surface. This article highlights some surprising post-launch errors and offers practical solutions.

Unpredictable 404 Errors

These are incredibly frustrating, skewing SEO data and making accurate analysis impossible. We experienced this during a JavaScript library update, encountering random 404s in our tools and Google Search Console, despite manual checks showing 200 status codes. The root cause often lies in server-side issues like rate limiting, but misconfigured caching, inconsistent DNS resolution, or load balancer errors can also be culprits. Server log analysis is crucial for identifying the problem; without it, troubleshooting is essentially guesswork. Ensure your SEO team has access to and understands server logs to effectively communicate issues to developers.

Elusive 500 Errors

While seemingly similar to random 404s, 500 errors usually have different origins. Even SEO crawlers can trigger them, particularly if crawling limits are exceeded or database queries are inefficient. Overloaded database servers, inadequate caching, and slow load times are common culprits. Again, access to server logs is paramount for diagnosis and resolution.

Resource Loading Inconsistencies

This issue manifested as a mysterious drop in rankings and traffic after a design revamp. Pages loaded correctly for users, but Google Search Console showed broken styling in its inspection tool. The problem, discovered months later via the browser console, was a script loading out of order, causing inconsistent rendering for Googlebot. This highlights the importance of checking browser console messages and considering the order of resource loading.

Non-Existent URLs Resolving as 200

While investigating 404s, we discovered non-existent URLs resolving with a 200 status code, flagged as duplicate content in Google Search Console. This poses SEO and security risks: search engines index irrelevant content, wasting crawl budget, while creating a potential vulnerability for malicious actors. Regularly check for this, focusing on programmatically or dynamically generated pages.

Hreflang/Canonical Tag Errors Targeting Non-Existent Pages

Managing hreflang tags on multilingual sites is complex. We mistakenly set the x-default hreflang to a non-existent English page. Incorrect hreflang tags confuse search engines. To prevent this, create detailed site-specific checklists and manually test localized pages across different templates.

JavaScript Rendering Failures for Bots

JavaScript-driven content visible to users but not bots is a common problem. If a widget requires user interaction, bots might miss it. Compare JavaScript-enabled and pure HTML crawls, and manually check if elements are present in the rendered HTML source.

Loss of Tracking Data

While seemingly unrelated to SEO, the loss of tracking data from paid campaigns due to improper handling of URL parameters during the migration significantly impacted remarketing efforts. Cross-team collaboration is key to preventing this.

Vanishing Pages

A third of blog posts disappeared after external DNS activation, highlighting the importance of pre-migration audits and using crawler comparison modes to detect discrepancies.

Admin Panel Overload

Heavy crawling overwhelmed the CMS admin panel, hindering content updates. Coordinate SEO crawls with content team workflows to ensure system resilience.

The Critical Importance of Post-Migration Monitoring

Website changes always carry risks. Some errors are obvious; others are subtle. Thorough planning, documentation, pre- and post-migration audits, and cross-team collaboration are essential for a successful migration. A successful migration is an ongoing process, not a one-time event.

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