The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

While development of magento application,  many times we get following error:

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

You need to delete  maintenance.flag file in root folder of magento to fix this problem.

When Magento is performing certain tasks, it temporarily creates maintenance.flag file which should be deleted automatically once processing  task get finished but sometimes this file don't get deleted and remain on root folder which causes above error. So, simply delete  this file to get back to development again!