GPG error: The following signatures were invalid: BADSIG 16126D3A3E5C1192 Ubuntu Extras Archive Automatic Signing Key

>>> Issue
W: GPG error: http://extras.ubuntu.com precise Release: The following signatures were invalid: BADSIG 16126D3A3E5C1192 Ubuntu Extras Archive Automatic Signing Key

<<< Fix #1 (NG)
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 16126D3A3E5C1192

<<< Fix #2 (OK)
sudo -i
apt-get clean
cd /var/lib/apt
mv lists lists.old
mkdir -p lists/partial
apt-get clean
apt-get update
This will rebuild the cache !