If you can't boot into Safe mode (I couldn't on a rMBP with 10.8.3), you can perform the following sequence to make it boot to Safe mode (source https://discussions.apple.com/message/21366643#21366643):

  cd /sbin
  sudo mv fsck_hfs fsck_hfs_orig
  sudo cp -p /usr/bin/true fsck_hfs
  nvram boot-args="-x -v"
  
  reboot, log in
  
  sudo nvram boot-args=""
   
  reboot
   
  cd /sbin
  sudo mv fsck_hfs_orig fsck_hfs