I have an apache box that runs a bunch of PHP and flat HTML sites. I have to set it to only use 10 processes, and to kill them every half hour, because they all gradually swell up to 35MB each (which I imagine is where they've loaded pretty much all the PHP on my server, independently of each other).

Without the number limit, or the kill policy, the server runs out of RAM and crashes. (it's only a cheap one, with 512MB RAM.) Luckily it's a very low traffic set of sites, so these limits don't break the experience. I'm glad I didn't have to solve this problem any deeper!