In this post, there is a link to a Google Docs document (on a line by itself, so oneboxed). However, the onebox isn’t visible for me (Firefox 45) – there is only a piece of white space. I had to look at the source for the post to see what the intention was.
Can other users see the box? If so, in which browser(s)?
Can’t see it either in Chromium 48.0.2564.116
Ah, I already wondered what’s wrong with this post. Looks like the request URL does not want to be framed, it’s sending an X-Frame-Options: SAMEORIGIN header, leading to:
Load denied by X-Frame-Options: https://docs.google.com/document/d/1Qi-5hDrjvhSBikI8Qku8tors6Z8sP43icDp-pUvDEjQ/pub?embedded=true does not permit cross-origin framing.
That’s a shame. I don’t really have a clue as to whether there’s anything we can do to fix this. 
Ok, I found something about it here:
So the trick is that the document needs to be published (inside Google Docs File -> Publish to the web) and not just shared. I did not even know about that feature before. Unfortunately the onebox also triggers for just shared links Also it does not provide any link to the published document 
Example:
https://docs.google.com/document/d/1kq3ci5beAUwlmWZUKuuVYCPIxcFEuPwKJk8M9XMvGT8/pub
Any chance you want to pursue this on meta.discourse? 
Already typing… 