On Linux Mint I got this error message while trying to run sudo apt-get update. Here is the full error message: W: GPG error: http://extra.linuxmint.com rosa Release: The following signatures were invalid: BADSIG 3EE67F3D0FF405B2 Clement Lefebvre (Linux Mint Package Repository v1) <root@linuxmint.com>

This is what fixed it: sudo rm -rf /var/lib/apt/lists/*

After removing those, sudo apt-get update ran just fine.