👨‍🏫 Tutorial How to Fix: The following signatures couldn't be verified because the public key is not available:

[Draft]

If you suddenly encounter this error/similar error:

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2B3F92F902D65EFF

Here's the solution:
Re-download the missing keys from the Ubuntu key server.
In my case:

Reading package lists... Done
W: GPG error: You do not have permission to view the full content of this post. Log in or register now. precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2B3F92F902D65EFF

Here is the command to add the missing key mentioned in the error message.

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 2B3F92F902D65EFF

So I take the missing key 2B3F92F902D65EFF and ask the keyserver.ubuntu.com to added it to the list of keys I have in apt.