[tsimionescu]

You're right, I completely forgot what it meant... The loop I wrote was basically equivalent to :

  for (int i = 0; i < nums.size(); ++i) {
    nums[nums[i]] = rng();
  }

Which happens not to go out of bounds simply because nums[i] is initially 0 for every i in the range - so it sets nums[0] to a different random number 1000 times.