The keys on the keyboard randomly stop working. This appears to be due to a design flaw that allows even a small amount of dust to interfere with the key action mechanism. Apple posted a 'fix' involving holding the laptop at odd angles and blowing compressed air under the keys to attempt to get them working again. Anecdotally, several HN users have reported having their keyboard replaced multiple times under warranty due to this issue.

Dust or debris can get under the keys and cause them to fail. There are also complaints that some keys don't register as well as others, and that some keys have a different feel than others (in terms of sponginess, etc.)

Like other users said, even a small piece of dust can cause keys to fail. The keyboard is not user-servicable and can only be repaired by taking the laptop apart (ungluing the case).

The keys have so little travel on them that even a spec of dust can lock them in place.