These kind of links worked not very long ago:
http://e-liquid-recipes.com/recipe/1121597/Banana%20Cream%20Pie%20(Gmix)
Producing oneboxes like this:
But not any longer - I tried inputting them on:
But I couldn’t see what was wrong… Especially when I compared the output to Discourse’s own output…
Can someone point me in the right direction?
No insight on this? I still doesn’t work on my site (which used to work fine)…
It’s only been 2 days. You have to give us a little while to get to things 
haha sorry, you’re right - it did prompt me to investigate further.
It looks like the onebox’er sends a HEAD request, which would explain it not working on my site, as I only support GET and POST. Doing a HEAD results in a 501 Not Implemented. When did this change?
Ah, this is a recent change to improve security with redirects.
I’ve been meaning to make it so that if the HEAD request receives a 501 it tries a GET. I will add that to my list.
Excellent. Now I know why, and that the problem is in very capable hands. Now I just have to ask my users to be patient Thanks! 
It’s fixed now:
Was quite involved. The fix wasn’t as simple as to try HTTP GET, but it also involved supporting cookies on redirects which our more secure redirector did not do. Both are supported now.
https://github.com/discourse/discourse/commit/53b95f009fc457f033427147ce34b4aa2e472f74
https://github.com/discourse/discourse/commit/369bb78f8e49c4cc4c3b585b40e4dc143a8b2669
https://github.com/discourse/onebox/commit/ea4a45bbf452bc3c40d470889daf21faa7fa6ba0