A trusted friend and I had ssh'ed into a Mac OS X Web Server (either snow leopard or lion) from two different locations. Then my password stopped working when trying to log in.
I can safely rule out the possibility that my friend changed my password, and I'm pretty sure another malicious user didn't do it. The only thing my friend did was send his ssh keypair to the server.
Somebody else set up the server so I am not sure how everything is configured, but is there a security measure employed by mac servers that kicks users who are logging in from two different IPs at the same time?