If haproxy is configured to use /dev/logger, it can sometimes print to stderr:
service haproxy restart
* Restarting haproxy haproxy
[ALERT] 093/183407 (2755) : sendto logger #1 failed: Resource temporarily unavailable (errno=11)
Reason is small udp queue, and non-blocking UDP sockets plus slow syslog cause those errors. In detail it explained
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