On CentOS 6 and CloudLinux 6 systems, the error "backlog limit exceeded" is logged either to /var/log/messages or to the console. The system may or may not be responsive.
This issue can happen for a number of reasons, primarily:
- Frozen Filesystem
- Limits too low
RedHat hs published an article to address the frozen filesystem issue.
RedHat has a premium article available for addressing the low limits, or there is a StackExchange post here addressing the same issue.