Here’s something to note after almost being locked out of an account.
Be careful about bash variable substitution when using ssh-keygen -N. Or better yet, don’t use ssh-keygen -N at all, preferring ssh-keygen -p PRIVATE_KEY_FILE.
The reason why is that the passphrase provided to -N can be modified by reason of variable substitution in bash. For instance, if you had the characters $? in your passphrase as provided to -N, they’ll be replaced with last command’s pid – good luck finding out what that was after trying to unlock your private key a few times.