Process table full up with ssh_agent processes created by Terminal

Question

Every time I start a new shell using Terminal, a new ssh_agent process is started. (It's PID is not stored in the environment variable SSHAGENTPID.) The problem is that this ssh_agent process lives on after the shell process completes.
Over time, as more new shells are started, the number of ssh_agent processes grows and grows. Eventually no new shell can be started. The following error is given:
login: fork: Resource temporarily unavailable
Some (but not all) applications will not start either (e.g. Cisco VPN Client).
The ssh_agent processes must be manually killed to rectify things without logging out.
What is wrong? Is this a known bug? How do I fix this?

Answer 1

Mystery solved.
I was confused because it seemed that one of my own or the standard login scripts must have been running ssh-agent every time, but I don't have a .bashrc nor a .bash_profile, and I have never changed the bash shell startup files. I have put some aliases into .bash_login, but I couldn't see how an alias command could be the culprit.
But after sleeping on it I went back and had a look at my .bash_profile aliases. There is one which read:
alias nopwd="eval $(ssh-agent) ssh-add"
which I had come up with because
eval `ssh-agent` ssh-add
cannot be put in an alias as written. It was only meant to be invoked manually, when and if required. However actually when this alias command in the .bash_login was parsed, it executed ssh-agent (resulting in an orphan process), rather than waiting until I typed nopwd. One reason I didn't realize my mistake earlier was because I was not asked for a passphrase - the ssh-add was not being executed.
BTW, typing
alias nopwd
didn't give
alias nopwd="eval $(ssh-agent) ssh-add"
rather it gave
alias nopwd='eval SSHAUTHSOCK=/tmp/ssh-h3nkOu3nb7/agent.1538; export
SSHAUTHSOCK;
SSHAGENTPID=1539; export SSHAGENTPID;
echo Agent pid 1539; ssh-add'
and typing nopwd during the shell worked fine as I had intended it to.
So, mystery solved. I've changed the alias to:
alias nopwd="eval \`ssh-agent\` ssh-add"
which works as intended, and no more ssh-agents popping up when I start a new shell. I could use a .bash_profile script to reuse an existing ssh-agent process, but I think I might start using SSHKeychain instead.