Random Instant 502 Bad Gateway Errors

[TheCrach]

I've just get a 502 Bad gateway. It's very random but instant ( the server do not take time to show the error message ).
This is strange because any logs appear in Nginx and PHP-FPM. And I've decrease the children value to 64.
I don't understand why this error appear !? For me, it seems that the child is restarted at nginx loading :/

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,91854,92161#msg-92161