I was trying to make apache 2.2 reject connections if MaxClients was reached, to do this I found the directive ListenBacklog.To test it, I configured apache in the following way:

<IfModule mpm_prefork_module>
    StartServers          5
    MinSpareServers       5
    MaxSpareServers      10
    MaxClients            10
    ListenBacklog        1
    MaxRequestsPerChild   0
</IfModule>

I've made a little script in JMeter to test this. The test launchs 50 users in 1 second (it requests a phpinfo page) but none is rejected, they all wait! I don't understand how this directive works... my operating system is Ubuntu.