Forum: NGINX When nginx will send Connection timed out error ?

I am using nginx as proxy server. I got following error once:
   upstream timed out (110: Connection timed out) while connecting to
upstream,

In what situtaion I will get this error? Exaple:
 1.  Upstream didn't accept connection within x time.
Is there any other case when I can get this error or is it only one case
when I will get this error?
 Example:
   1. Saving or reading to tmp file buffer was slow.
   2. Nginx was running out of connection?

Posted at Nginx Forum: 
http://forum.nginx.org/read.php?2,232970,232970#msg-232970