int returns_less_than_twelve(int input1, int input2) {
    int sum = 0;
    if (input >= 5) {
       sum += 5;
    } 
    if (input2 >= 5) {
      sum += 8;
    }
    return sum;
  }

The following test cases will pass and achieve 100% branch coverage.

  int x = returns_less_than_twelve(5,0);
  test_assert(x < 12);
  int y = returns_less_than_twelve(0,5);
  test_assert(x < 12);

However, this does not cover the entire input space so

  returns_less_than_twelve(5,5);
  test_assert(x < 12);

Will fail. However, branch coverage won't tell you that there's a hole in your test coverage.

Generally however, writing full branch coverage will find a lot of issues, and also cause you to really think through how your code works; but still, it doesn't guarantee correctness. If you want that you need to start bringing tools that either exhaust your input space (a function which takes 5 booleans can be exhaustively tested for correctness in trivial amounts of time), or you start modeling your chosen language well enough that you can use a mathematical prover to demonstrate that your program or function is safe on all inputs.

This of course requires you to come up with a definition of 'correct' or 'safe'. For the above program, it's clear how to define correctness. For things like "Don't let an unauthorized individual access this data or data that is derived from it in a way contrary to the desires of the owner of said data" it gets 'tricky' ;).