Say I try to post an image that is hosted from another forum. The other forum is set up to require login to view images, but I don’t realize that. I post it on Discourse, and it seems to work and I can see the image with no problem, because I’m logged into the other forum. I don’t know anything is wrong until other users tell me they can’t see the image.
However, because the server isn’t logged into the other forum, it can’t access the image to cache it. I believe this should result in an error of some kind that is visible to the user who posted the image.
Perhaps if the system account fails to cache an image, it replaces it with some kind of error/warning (should not remove the URL so the user can return and fix it somehow).
Example: I can see the following image. No one else will unless they are logged into the Duramax Forum.