This means that your OS X kernel cannot start more processes for your user ID. Maximum number of processes for each user:
To see the process limitation:
sysctl kern.maxprocperuid
To check how many processes you have running:
ps aux | grep username | wc -l
To find out how many zombie processes:
ps -v -u username | grep -v grep | awk '{print $2}' | grep Z | wc -l
To find out what it is:
for pid in `ps -v -u username | grep -v grep | awk '{print $1, $2}' | grep Z | awk '{print $1}'` do ps -o command="" -p $pid >> zombies.txt done
None of these processes can be killed, and cleaning will not work on my mac, 10.8.2.
You can temporarily increase the maximum number of user processes:
sudo sysctl -w kern.maxprocperuid=1024
Then in your .bash_profile or .bashrc add:
ulimit -u 1024
And change the limits of the Finders:
sudo launchctl limit maxproc 1024
But actually, it's time for coffee and give your Mac a reboot.