Today, I tried a "git pull" from a github repo, and to my surprise, was prompted to enter my SSH passphrase for my key. Normally, I don't have to type anything - that's the beauty of SSH keys.
After mucking about far too long, I finally realized that this meant that the
ssh-agent process on my Mac had died. Unfortunately, there seems to be no easy way to restart this from the command line (see this post). So, much to my chagrin, I rebooted my Mac. Damn, it's almost like having to use Windows.
Fortunately, rebooting reset whatever was going on with ssh-agent, and git started working properly again. Whew.