go/doc/next/6-stdlib/99-minor/net/http/66343.md

[ServeContent], [ServeFile], and [ServeFileFS] now remove the Cache-Control, Content-Encoding, Etag, and Last-Modified headers when serving an error. These headers usually apply to the non-error content, but not to the text of errors.

Middleware which wraps a [ResponseWriter] and applies on-the-fly encoding, such as Content-Encoding: gzip, will not function after this change. The previous behavior of [ServeContent], [ServeFile], and [ServeFileFS] may be restored by setting GODEBUG=httpservecontentkeepheaders=1.

Note that middleware which changes the size of the served content (such as by compressing it) already does not function properly when [ServeContent] handles a Range request. On-the-fly compression should use the Transfer-Encoding header instead of Content-Encoding.