If you come across this error after running aptitude update
W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available:
W: You may want to run apt-get update to correct these problems
due to a missing GPG key do the following to correct the problem
gpg --keyserver subkeys.pgp.net --recv 7D2C7A23BF810CD5
Replace the key number “7D2C7A23BF810CD5” with the one included in your error message
gpg --export --armor 7D2C7A23BF810CD5 | sudo apt-key add -
After that, the key will be added to a list and the error will not reappear.