Have you ever seen this dreaded message in your Apache/HTTPD /var/log/httpd/error_log?

[error] server reached MaxClients setting, consider raising the MaxClients setting

The error itself is slightly misleading.  I believe this happened to one of my servers, I found Apache was running still, and that you could telnet to port 80 but no respnose would be given.

For some reason my error log initially did not have the above error, but after a restart I saw it.  I'm assuming it was a DOS attack or increased traffic.

The "MaxClients" setting is not the only thing to take into consideration, if you follow the above you'll get an error message like this:

httpd: WARNING: MaxClients of 500 exceeds ServerLimit value of 10 servers,
 lowering MaxClients to 10.  To increase, please see the ServerLimit
 directive.

So make sure you edit /etc/httpd/conf/httpd.conf

And check the ServerLimit value first and set that to something like 100-500 and then set MaxClients