Website migrations are one of the most challenging aspects of SEO.
No matter how much experience you have in technical SEO, how detailed your plan is, or how thorough your checklist may be, unexpected issues can still arise.
That’s why post-migration monitoring is just as crucial as the migration itself – especially in the first month when hidden problems are most likely to surface.
This article tackles some of the most surprising post-launch errors I’ve encountered, along with practical tips on how to identify and resolve them before they cause serious damage.
Random 404 pages
This issue drove me crazy. It’s a nightmare for SEO testing because it skews every tool and report we rely on.
When you can’t trust the data, it’s impossible to know what’s actually broken or how it impacts performance.
During the post-migration phase of updating our JavaScript library, we noticed random 404 errors in our SEO tools and Google Search Console.
The strange part?
The affected pages weren’t consistent, and every time we checked manually, they loaded fine with a 200 status.
As a result, all other reports became unreliable, making proper analysis nearly impossible.
These random 404s often stem from server-side issues such as rate limiting, where the server denies access to bots after too many requests.
Other potential causes include:
Misconfigured caching.
Inconsistent DNS resolution.
Load balancer errors that occasionally route requests to an unavailable server.
Identifying the root cause requires detailed server log analysis to track bot request and response patterns.
And here’s the biggest lesson I learned: Without access to server logs, you’re fighting this battle blind.
Ensure your SEO team has access to the necessary server log tools and, at the very least, understands the basics of how they work.
Monitoring bot activity logs can help you demonstrate the issue to developers. Without them, you risk getting stuck in endless debates over the accuracy of SEO tools.
Dig deeper: Website migration checklist: 11 steps for success
Random 500 pages
At first glance, this error looks similar to random 404s, but the cause is usually entirely different and just as difficult to diagnose.
Even SEO tools like Lumar and Screaming Frog can inadvertently trigger these 500 errors while crawling.
Years ago, one of the websites I worked on had a strict rule: no crawling on weekends and no exceeding three URLs per second.
Every time we increased our crawling limits, the database server struggled, slowing down the entire site – or worse, crashing it.
These errors often result from complex database queries overloading the server or improperly configured caching.
Without proper caching, each request is processed individually, compounding the strain and leading to slow load times or intermittent crashes.
And once again, the solution starts with server log access. Without it, you’re just guessing.
Incorrect resource loading
This was one of those moments where I felt like a digital Sherlock Holmes.
The migration had been completed before I joined the company, and I first noticed the issue during an initial technical audit.
The first clue?
A mysterious drop in rankings and traffic shortly after the migration.
There had been a Google update around the same time, so I couldn’t immediately link the decline to the migration.
To complicate things further, this wasn’t a full migration, just a design revamp.
On the surface, everything seemed fine. Pages loaded correctly, and styles and JavaScript worked perfectly for users.
Yet, in Google Search Console’s inspection tool, the same pages often appeared broken and unstyled.
The issue was inconsistent, making it nearly impossible to replicate in front of the dev team.
As a new team member still building trust, convincing them there was a deeper problem wasn’t easy.
In hindsight, my mistake was not checking the browser console earlier.
Three months later, a single browser console message finally revealed the root cause: a script was loading out of order.
Due to caching, Googlebot sometimes saw the website correctly and other times didn’t, explaining the erratic behavior.
It was a tough reminder that small technical details – like the sequence of resource loading – and overlooking an obvious diagnostic step can significantly impact SEO performance.
My key tip: Check your website in different browsers and carefully review the error and warning messages in the console.
If you’re unfamiliar with developer terminology, consult an independent expert or even multiple AI tools for explanations.
Non-existent URLs
While investigating those frustrating random 404 errors, I stumbled upon another issue almost by accident.
While reviewing Google Search Console’s report on pages discovered but not indexed, I noticed an unusual pattern – several non-existent URLs appearing under certain sections, marked as duplicate content.
Instead of returning 404 errors as expected, these URLs resolved as normal pages with a 200 status code.
This type of error presents two major risks:
From an SEO perspective, search engines treat these URLs as legitimate, potentially indexing irrelevant or duplicate pages, wasting crawl budget, and harming rankings.
From a security standpoint, it creates a vulnerability – malicious actors could generate thousands of random URLs, overloading the server.
Unfortunately, this issue is difficult to detect before it becomes a real problem. In my case, I was just lucky.
Don’t wait to stumble upon it. Make sure to:
Regularly check whether sections of your site allow non-existent URLs to resolve with a 200 status.
Build a list of key sections and test them monthly with your crawler. Even minor backend changes – not just full migrations – can trigger this issue.
Prioritize pages generated programmatically or dynamically, as they are the most common culprits.
Managing hreflang tags on a multilingual website is challenging, and even small mistakes can cause big issues.
On one website I worked on, we typically created pages in English first and then localized them.
However, in some cases, only a local version existed, and the hreflang x-default was mistakenly set to an English page that didn’t exist.
Incorrect hreflang tags confuse search engines, which rely on them to identify the correct language or regional version of a page.
When these tags are wrong, search engines may struggle to understand the site’s structure or ignore the hreflang implementation entirely.
Normally, we would have caught this in our migration checks.
But at the time, we were buried in troubleshooting random 404 errors.
We also made the mistake of not manually testing localized pages across different templates.
To prevent this in future migrations: