While working on the book Go in Practice I found an annoying little problem in the Go built-in file server. The error pages, such as a 404 Not Found response, had no ability to be customized. What is does send are text, not HTML, with an error message in English.
If you're into web development than customizing these pages is needed. The customizations can be made to make the pages useful, to put hidden gems in them (for example, a video game), or simply to have the design match their site.