According to a thread on MacRumors some people have reported problems with the new aluminum MacBook's battery cover coming loose.
One user writes:
The cover was fine at first, but after i took it off for the first time, when I put it back on it didn't go as tight, and started moving around creating gaps in the front and back. Also, when I go to sit it down it clicks (the cover is hitting either the battery or the frame...)
In addition to the loose battery cover, some users have reported slanted function keys and my personal favorite - extra "m" keys.
(Picture courtesy: Engadget)