ANSWER:

“HTTP Error 503 – Service is unavailable” error occurs if your application pool fails due to the error made to your application more than Maximum Failure limit allocated to your apps in iis.

Resolution for it

1. Open IIS Manager.
2. Figure out your application pool and click the advanced setting of your pool.
3. Once you click the advanced setting, it will open and window.
4. Here under “Rapid Fail Protection” you can find the “Maximum Failure” increase it from default 5 to 50.

After modifying the Advanced Setting, recycle the apps.

by Mughil from http://serverfault.com/questions/343727