This is merely a "for your information" for those using the Amsterdam Imgur server and not seeing all images. Today, that Amsterdam server gives me an error for many (but certainly not all) images. It has already been reported to Imgur.
For example, some image gets me an HTML error page:
$ curl -verbose http://i.stack.imgur.com/yWFzG.png > test.png [...] HTTP/1.1 504 Gateway Timeout Content-Type: text/html Server: ECAcc (ams/489C) Content-Length: 357
For the very same URL, Web-Sniffer reports a different server, which does return the image:
HTTP/1.1 200 OK Content-Type: image/png Server: ECAcc (fra/D4A4) X-Cache: HIT Content-Length: 64960
Even just going to http://i.stack.imgur.com fails for me.
I'm sure Imgur will fix this.