I just casually went ahead and saved my partial as a page resource for a post, say content/posts/foo/partial.html. However, I think Hugo started evaluating that HTML file, because of which my builds are now failing by stating:
~/G/bitbanged.com main !2 ?1 ❯ hugo server ✘ 255
Start building sites …
hugo v0.93.1+extended linux/amd64 BuildDate=unknown
Error: Error building site: "/home/subaru/GitHub/bitbanged.com/content/posts/foo/partial.html:1:1": unmarshal failed: invalid character '{' looking for beginning of object key string
Built in 18 ms
Page resources that are content files (md, adoc, org, rst, html) are processed. As you have found, that can be a problem if you want to insert a file’s .Content into something. I’ve handled this in the past by using a global resource instead (put it in the assets directory).