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.

Missing Dock background in OSX Mountain Lion [click for full size]

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).