As a security precaution, GitHub automatically deletes SSH keys that haven't been used in a year. On CircleCI this results in the checkout step failing with the following error:
Either git or ssh (required by git to clone through SSH) is not installed in the image. Falling back to CircleCI's native git client but the behavior may be different from official git. If this is an issue, please use an image that has official git and ssh installed.
Cloning git repository - git@github.com:org/repo.git
error cloning repository: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain
This error does not tell the user the deploy key has been deleted, which can cause confusion. If the project and/or job was flagged as not having a deploy key that would allow the user to fix the issue without having to investigate.