Just a second ago I logged in as guest in my mom’s 13″ 2009 MacBook (model MacBook6,1) and I noticed that Dock is missing its background.
Dock has no bacground. As you probably know, Dock is also responsible for Mission Control and Launchpad (it’s like Mission Control for Mission Control), as a result, Expose also has no background. Neither has Launchpad. Even Notification Center is buggy. What is the solution? Yes, you guessed it.
$ killall Dock
It relaunches Dock and everything connected with it – Launchpad, Mission Control, Notofication Center. Everything should work fine after the killall. Just to be sure, here’s the video I made for you to demonstrate what’s going on (and it’s a proof that I didn’t photoshop the screenshot :P).