After updating the kernel (on RHEL 3) to 2.4.21-27.ELsmp from 2.4.21-20.ELsmp, Apache started giving Internal Server Errors for all PHP pages (and presumably other types of scripts as well). A look at the error log revealed lines such as: Setting MaxClients very low (i.e. less than 20) seemed to reduce the frequency of these errors, but did not eliminate them entirely. For now I've switched back to the old kernel, but this is a temporary fix because of known vulnerabilities in that version. My best guess is that the new kernel is enforcing some kind of limits on the number of child processes a process can have that the old kernel is not. Perhaps someone with more experience than me could shed some light on this?