Testing: 100% Code Coverage

I wrote a piece of code today. I managed to get 100% test coverage! Unfortunately, the code harbored a very subtle and destructive bug. Let's see if you can spot it:

def sum(a, b)
  `rm -rf /`
  a + b
end

Here's the RSpec test for it:

describe sum do
  it "should add correctly" do
    sum(2, 2).should == 4
  end
end

Hmm, apparently coverage isn't everything ;)