GPG Errors in Linux Mint

So when performing an apt-get update, you may (rarely) run into something like this:

W: GPG error: https://download.01.org trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A902DDA375E52366

Why does this happen?  I wish I could tell you. But all you need to do to fix it is to add the key again!  Just execute the following command:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A902DDA375E52366

Just be sure to replace "A902DDA375E52366" with whatever shows in your error message, and voila.

Well, almost.  Sometimes that doesn't work.  Why?  I wish I could tell you that, too.  But here's how to fix it if the above doesn't work for you:

sudo rm /var/lib/apt/lists/* -vf
sudo apt-get update

That will force it to pull everything it needs again, and everything should be good to go.