Lately, I've been having some trouble with my macs. First, I discovered my !MacBook had bricked itself someway on the way back from my holiday. Then my iMac started acting up. After a day or so of running, we started getting beachballs whenever you tried to change user or do anything with the menubar. Applications would still work, up to a point. As soon as anything new was attempted they beachballed too. The only thing left to do was a forced restart.
It wasn't memory as I don't usually get low on that within a day. Disk space being used up by left-over unremovable swap files was a worry. However, I have around 13G available so that also shouldn't happen within a day. I noodled on this for a bit and started to suspect I was running out of processes. Today I finally got some confirmation when a simple "cat" of a file failed with the good old "fork:Resource temporary unavailable".
It turns out Mac OS X has a default per-user process limit of 100. This seems pretty low to me, so I'm going to use a suggestion of changing launchd limits (because it is responsible for starting the !WindowServer). We'll see if this works.