Flash Player 11.8 install fails with error code 30 listed in console
I completed the installation by running the installer as root from the shell, under its mountpoint at /Volumes. The full path to the installer executable's directory is /Volumes/Adobe Flash Player Installer/Install Adobe Flash Player.app/Contents/MacOS

This thread in Adobe forums describes the underlying problem - the current user can't write to a directory the installer needs. Instead of chmod'ing the folder, I already had root enabled, and went from there successfully.

Update: Commentor is right, root is too risky. Follow the chmod route.