Update Faker allows you to "fake a system update", it's the perfect way to prank your friends, family members or colleagues. Especially when they're working on something rather important.
While they are away from their PC navigate to this website (updatefaker.com) and click on one of the appropriate themes below. Once they arrive back at their computer they will almost certainly start to freak out.
You decide how long you want to let them suffer in frustration before telling them it's just a prank website.