If you’re seeing the above error on Mac OS X, see pdm’s Mac OS X hint for the quick fix. For a more thorough explanation, check out John Bellardo’s page about the fork and vfork problem in Darwin. John’s page also has a neat kernel extension that somewhat fixes the bug, but it didn’t work for me with Panther (10.3.5), and I was after a really simple fix.

Update: If you’re running Mac OS X 10.4 Tiger, there’s a much easier solution that involves (you guessed it) editing a launchd configuration file.