First test pass but the second one fails - I do not understand why. The only difference is that the second test ticks 200ms (which is more than needed), while the first one only ticks 100ms.

gistfile1.js

var assert = require('assert'),
    sinon = require('sinon');

describe("Testing sinon useFakeTimers", function() {
  it('sinon faking timer should work with small intervals', function() {
    var clock = sinon.useFakeTimers();

    var spy = sinon.spy();
    setInterval(spy, 100);
    assert(! spy.calledOnce);
    clock.tick(101);
    assert(spy.calledOnce);

    clock.restore();
  });

  it('sinon faking timer should work with bigger intervals', function() {
    var clock = sinon.useFakeTimers();

    var spy = sinon.spy();
    setInterval(spy, 100);
    assert(! spy.calledOnce);
    clock.tick(200);
    assert(spy.calledOnce);

    clock.restore();
  });

});