I just bought the new Mac mini 2.5 model 3 days ago, and I have been loving it. But yesterday I decided to change my username on my Mac, and may have found a pretty bad glitch in Lion. After changing my username, I come back to my Mac a few hours later and the screensaver has come on. So as usual I go to move the mouse and enter my password, but it says that my password is wrong! It was definitely not wrong, because I had entered this password countless times already and had written it down in case I forgot. I tried entering it multiple times and it was always wrong. Then I noticed that it was showing my username as my old username before I changed it. So finally I just had to restart the Mac. Once it came back on it showed my updated username, and I was able to log in with my password as usual. So be sure that if you change your username, you should restart your Mac immediately.