I found this article rather interesting:


They crack passwords by measuring the time it takes for a computer to respond to a login request. On some login systems, the computer will check password characters one at a time, and kick back a "login failed" message as soon as it spots a bad character in the password. This means a computer returns a completely bad login attempt a tiny bit faster than a login where the first character in the password is correct. By trying to log in again and again, cycling through characters and measuring the time it takes for the computer to respond, hackers can ultimately figure out the correct passwords.
This has been around as a concept for 25 years or so, but a couple of guys reckon that it is not so difficult as previously thought.