$ mocha --version
1.7.0

$ cat testing/a.js
describe('Initial test', function() {
    before(function() {
        console.log('initial before');
    });

    describe('test will fail', function() {

        it('will fail', function() {
            throw new Error('failed');
        });
    });

    after(function() {
        console.log('calling after');
    });
});

$ cat testing/b.js 
describe('Second test', function() {
    before(function() {
        console.log('second before');
    });

    describe('second test does not matter', function() {

        it('this test should not run', function() {
            throw new Error('I do not matter');
        });
    });

    after(function() {
        console.log('calling second after');
    });
});


$ mocha --bail testing/

initial before
․calling after
second before // SHOULD NEVER BE CALLED
calling second after // SHOULD NEVER BE CALLED

✖ 1 of 2 tests failed:

1) Initial test test will fail will fail:
    Error: failed