When you try to access ubuntu extras you might see following GPG error
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://extras.ubuntu.com maverick Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/maverick/Release
Open the terminal and run the following commands
gpg –keyserver keyserver.ubuntu.com –recv 3E5C1192
gpg –export –armor 3E5C1192 | sudo apt-key add --
sudo apt-get update