I wrote an article that was about 404 pages on Zeit.co and how to set them up. At the time it was undocumented and hard to find.
At the start of the article, I had a section about different 404 pages around the internet. A company reached out to me to ask if I would include there 404 page in the article.
I ended up splitting the article into two, one about different 404 pages, and another about specifically configuring Zeit.co.