A couple days ago, my brother was playing around with one of the other students' phone, trying to guess the phone's password for fun. He didn't get it, but he got close, apparently.
Today, when the guy entered his password a few seats away from my brother, my brother asked to try guessing again.
A moment later, the phone was unlocked.
So the guy changed his password. My brother got that one on the first try.
Here's a hint for you: if your telephone beeps in a different tone for each number, your password is not secure. People who know music can get it just by listening to you type it in.