As recently as yesterday, I was able to connect to our development instance via my local ssh client, using one of several ssh keys listed under sshKeys in the instance's custom metadata (the old-fashioned way). ("Block project-wide ssh keys" is checked). Also that afternoon, while connected, I was doing something where a second ssh session would be convenient. I tried opening a second connection through the local ssh client, and got "Permission denied (publickey)." I tried opening a second connection from the instance console page, and it wouldn't connect, either. Meanwhile, the ssh session I was using continued to work just fine.
This morning I can't get an ssh session on that instance AT ALL.
The only thing I can remember doing yesterday, that was the least bit unusual, was a chmod on my home directory, setting it to "777."
I tried yanking out the sshKeys metadata, and trying the console ssh again. No joy. Then I looked at the serial port 1 log, and noticed this, with timestamps showing GMT for approximately when I tried it:
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
Nov 28 21:26:49 bitnami-trac-dm-87ea google-accounts: INFO Removing user <redacted>.
I can get to the password prompt on the serial port, but I've never set myself up with password access, to the best of my recollection.