Today when I tried to boot a VM I got a scary message:

> vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Clearing any previously set forwarded ports...
==> default: Clearing any previously set network interfaces...
==> default: Preparing network interfaces based on configuration...
    default: Adapter 1: nat
    default: Adapter 2: hostonly
==> default: Forwarding ports...
    default: 22 (guest) => 2223 (host) (adapter 1)
    default: 80 (guest) => 8080 (host) (adapter 1)
    default: 4444 (guest) => 4444 (host) (adapter 1)
    default: 3306 (guest) => 3306 (host) (adapter 1)
==> default: Running 'pre-boot' VM customizations...
==> default: Booting VM...
==> default: Waiting for machine to boot. This may take a few minutes...
    default: SSH address: 127.0.0.1:2223
    default: SSH username: vagrant
    default: SSH auth method: private key
    default: Warning: Remote connection disconnect. Retrying...
    default: Warning: Authentication failure. Retrying...
    <snip>
    default: Warning: Authentication failure. Retrying...
Timed out while waiting for the machine to boot. This means that
Vagrant was unable to communicate with the guest machine within
the configured ("config.vm.boot_timeout" value) time period.

If you look above, you should be able to see the error(s) that
Vagrant had when attempting to connect to the machine. These errors
are usually good hints as to what may be wrong.

If you're using a custom box, make sure that networking is properly
working and you're able to connect to the machine. It is a common
problem that networking isn't setup properly in these boxes.
Verify that authentication configurations are also setup properly,
as well.

If the box appears to be booting properly, you may want to increase
the timeout ("config.vm.boot_timeout") value.

The odd part was if I tried to manually SSH into the VM I received a password prompt which isn’t supposed to happen.

> vagrant ssh default
vagrant@127.0.0.1's password:
Welcome to Ubuntu 12.04 LTS (GNU/Linux 3.2.0-23-generic x86_64)

 * Documentation:  https://help.ubuntu.com/
New release '14.04.1 LTS' available.
Run 'do-release-upgrade' to upgrade to it.

Welcome to your Vagrant-built virtual machine.
Last login: Mon Mar 21 20:28:37 2016 from 127.0.0.1

To fix this problem I needed to “reset” the authorized_keys file to the default:

wget https://raw.githubusercontent.com/mitchellh/vagrant/master/keys/vagrant.pub -O ~/.ssh/authorized_keys
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
chown -R vagrant:vagrant ~/.ssh

After another vagrant command Vagrant automatically replace the default file with one specific to my computer and we were back off to the races.

> vagrant halt
==> default: Attempting graceful shutdown of VM...
    default:
    default: Vagrant insecure key detected. Vagrant will automatically replace
    default: this with a newly generated keypair for better security.
    default:
    default: Inserting generated public key within guest...
    default: Removing insecure key from the guest if it's present...
    default: Key inserted! Disconnecting and reconnecting using new SSH key...

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