I don't know if this is the same as what you're seeing, but there's a feature of Arc application servers that does this. It's meant for rate-limiting people who might be attempting denial-of-service attacks against the site:
The default settings throttle anyone who's doing 30 requests in 10 seconds. Each page load does 5 or 6 requests to get images and such, so we can get throttled if we reload or follow links 7 times in 10 seconds. When I try it on Arc Forum, it seems to be doing exactly that.
At Hacker News... if I go a few pages into the recent comments and hit F5 approximately 30 times in 10 seconds, I get an Nginx error page. Hacker News probably lets Nginx serve its static files rather than having every request go through the Arc server's throttle, but otherwise its configuration seems to be pretty similar.
Maybe you're navigating 7 times in 10 seconds by hand, but there are other possibilities too. If you're on the same external IP address as others who are accessing the Arc Forum, you might all be under a single rate limit.
Hmmm, that's interesting but I don't think it's what I'm running into. I get it on a first visit. I don't think my housemates are visiting the arc-forum at all . . .
