In my config, within http section, I have:

limit_conn_zone $binary_remote_addr zone=addr:10m;
limit_conn addr 3;

which I interpret as, setting a memory zone of size 10 megabytes for keeping states of connected IPs and restricting 3 concurrent connections per IP at a time. If the size of the zone is exhausted or the limit per IP is breached, Nginx will return 503 as response.

I have them before upgrading to 1.2.8. During testing today, I found several 503 responses due to these settings, for example:

'2013/04/12 15:38:48 [error] 5888#0: *352 limiting connections by zone "addr", client: 127.0.0.1, server: static.mysite.com, request: "GET /js/jquery.reject.min.js HTTP/1.1", host: "static.mysite.com", referrer: "https://www.mysite.com/blah/blah?var=blah"

The client IP is 127.0.0.1 here because Nginx is behind a HAproxy.

The test I did was launching several requests which requests for delivery of some JavaScripts from Nginx. Some of these requests return 200 OK, but a number of them failed with 503s. The same test I did before with Nginx 1.2.7 did not result any 503 responses. Any idea what might have caused this? Is it because the HAProxy in front of Nginx? Thanks!