Update: It does look like that Blogger has finally taken action. They have disallowed the /related-content.g?q= in robots.txt file. So, it is no longer anything worth worrying about.
Some of them have even asked the same question by commenting on this blog. Despite knowing a solution, I did not tell them because of my reluctance and hope that Google would probably solve this error, which seems very unlikely (like most errors on Blogger).
The reason for my reluctance was that I was not sure if it was in agreement with Blogger's Terms of Service, violating which, will surely result in a blog being deleted. I did not wish that Google deleted other people's blogs due to me. Today, I re-read the Terms of Service and came to know that it did not mention anything about this "hack". Although, it is in Google's discretion to change the terms any time to suit their needs, it is probably safe for now to remove the script causing the error. To remove the script, follow the steps given below.
How to remove the related-content.g?q= 404 Error
- Login to Blogger.
- Click on Design.
- Click on Edit HTML.
- Search for </head> in your template.
- After locating it paste the following code immediately after the </head>.
- <!-- <body><div></div> -->
- Click on Save Template.
- An error message like the one given in the image might be shown
- Click on Delete Widgets.
What does this do?
How long does it take to work?
Well, it depends on your blog's crawl rate. If your blog gets crawled quickly, Webmaster Tools might remove this error in a week. If that is not the case, it can take up to a month.
P.S: As an additional benefit, deleting the Navigation Bar widget will reduce the number of errors and warnings in your blog's HTML validation through W3C.