In a recent episode of Google’s “Search Off the Record” podcast, an important SEO issue that could cause web pages to disappear from search results was discussed. Allan Scott from Google’s Search team shared insights on the formation of “marauding black holes,” which occur when error pages are incorrectly grouped together.

Google’s algorithms may mistakenly cluster error pages that appear similar, leading to regular pages being included in these groups. As a result, these pages may no longer be crawled, potentially causing them to be de-indexed—even after the errors are corrected. The podcast covered the causes of this issue, its impact on search traffic, and the steps website owners can take to prevent their pages from being lost in such black holes.

How Google Handles Duplicate Content
To understand the concept of content black holes, it’s important to first grasp how Google handles duplicate content. According to Scott, this process occurs in two stages:

1. Clustering: Google groups pages with identical or very similar content.
2. Canonicalization: Google then identifies the most appropriate URL from each group.

Once pages are clustered, Google stops re-crawling them, conserving resources and avoiding redundant indexing of duplicate content.

How Error Pages Lead to Black Holes
The issue of black holes arises when error pages, such as those showing generic “Page Not Found” messages, are clustered together because of their similar content. Regular pages that experience occasional errors or temporary outages may inadvertently become part of these error clusters.

Google’s duplicate content handling system prevents the re-crawling of any pages within a cluster, making it difficult for misclassified pages to escape the black hole—even after the initial errors are fixed. As a result, those pages could be de-indexed, causing a decline in organic search traffic.

Scott explained:
“Only the pages that are at the top of the cluster are likely to get out. This is a concern for sites with transient errors… If those errors prevent the page from loading, we may view the page as broken.”

How to Avoid Black Holes
To avoid the problem of duplicate content black holes, Scott recommended the following steps:

1. Use Correct HTTP Status Codes: Ensure error pages return the proper status codes (such as 404, 403, and 503) rather than a 200 OK status. Pages marked with a 200 OK status are more likely to be grouped with other pages.
2. Craft Unique Content for Custom Error Pages: For custom error pages (common in single-page applications) that use a 200 OK status, ensure they contain unique content to prevent these pages from being grouped with errors. For instance, include the error code and a description in the page content.
3. Be Careful with Noindex Tags: Avoid using noindex tags on error pages unless you want them permanently excluded from search results. This tag has a stronger effect than using error status codes alone, indicating that you want the page removed from Google’s index.

Implementing these practices will help prevent regular pages from being mistakenly grouped with error pages, ensuring they remain indexed by Google. Regular monitoring of your site’s crawl coverage and indexation is also crucial for identifying and resolving potential duplication issues early.

Google’s “Search Off the Record” podcast shed light on a significant SEO concern where error pages are mistakenly categorized as duplicate content. This can lead to regular pages being grouped with error pages and removed from Google’s index—even after the errors are fixed.

To avoid issues with duplicate content, website owners should:

1. Ensure error pages return the correct HTTP status codes.
2. Create unique content for custom error pages.
3. Keep an eye on their site’s crawl coverage and indexation.

Following these technical SEO best practices will help maintain a website’s visibility in search results and prevent accidental de-indexing.

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