node's process.nextTick() is used by a lot of libraries (q, for instance) but isn't patched by zone.js:

require('zone.js');
Zone.current.fork({}).run(() => {
  Zone.current.foo = 'bar';
  process.nextTick(() => {
    console.log(Zone.current.foo); // undefined
  });
})

process.nextTick was basically setImmediate before setImmediate existed. Now it's like setImmediate except it gets scheduled in a higher priority queue.