Quite recently I came across a very interesting issue: while configuring passwordless SSH (it's public key based, so depending on you have it configured it may not be completely passwordless) access to some of my VPS servers, I found that the same keypair just wouldn't work on one of the servers.
Not only that, but the behaviour was quite bizzare: upon my first attempt to connect the public key would get rejected and a regular password would be requested by the ssh session. But once I successfully logged in with my password, any subsequent ssh connections would happily authenticate by my public key and would let me in without a problem.
Those of you using home dir encrypiton in Ubuntu are probably smiling right now! 🙂 But becase I have never consciously configured or used this feature, it took me a good few hours to troubleshoot the issue and come up with the fix.