When I run Node.js I found the error like this :

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: listen EADDRINUSE
    at errnoException (net.js:901:11)
    at Server._listen2 (net.js:1039:14)
    at listen (net.js:1061:10)
    at Server.listen (net.js:1135:5)
    at Object.<anonymous> (/home/ubuntu/nodes/app.js:59:24)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)

That is a weird undetected error until I found that its probably caused by port setting. 

I change the Port from this :

app.set('port',process.env.PORT || 2000);

to this:

app.set('port',process.env.PORT || 8000);

And I finally can get rid of that error. If you have the same issue, you might need to try this one.