When does nginx return a Bad gateway (502)?

[fastcatch]

I have an application behind nginx (Rails on Unicorn if it matters) which
listens on a UNIX socket.

It all works nice, especially when load is low.  When there is some load on
the server though (say 70%), I randomly(?) get a bunch of 502 responses --
usually in batches.

Thus the question: when (under what conditions) does nginx send a 502
response?

Thanks in advance,

fastcatch

PS: The app's worker processes do not seem to be irresponsive, slow, blocked
for IO or anything else I have been able to identify as a possible cause. 
Thus I want to understand more the nginx side so as to find the root cause
-- which I believe is on the other side (or some config tweak needed
soemwhere).

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,237573,237573#msg-237573