Do you think after the Dyn outage everyones sysadmins are running round adding redundancy, too worried to trust the uptime of their site in the hands of just CloudFlare?
[1] http://nickcraver.com/blog/2016/02/17/stack-overflow-the-arc...
As you can whitelist Tor traffic in Cloudflare, it seems to be down to these 503 errors (edge to origin). But haven't heard that before, so not sure if it's a problem that occurs more often.
For Imgur, I could imagine that purging by cache tag is just too restrictive at Cloudflare (the limit is very low, even for Enterprise clients). Fastly doesn't have a limit there, they encourage you to cache everything and purge where needed. Makes it much easier to cache APIs and HTML pages.
Source:
https://meta.stackoverflow.com/questions/323537/cloudflare-i...
Pretty sure the captcha can be switched off for enterprise clients (which I presume SO would be).
Regarding caching, SO's caching is extremely aggressive. It's especially problematic when editing answers more than once, because when you click edit you'll be presented with a cached copy of your answer, potentially excluding your latest revisions. So you have to refresh the edit page and then edit the answer.
It might seem like an edge case, and it certainly teaches you to be meticulous about answering lest you have to go through the refresh hell, but it's not what some would designate as 'good UX'.
I am never surprised by SO's reports about how they run their busy website from a mere handful of machines precisely because of the caching they do.