Login failed for user 'NT AUTHORITY\NETWORK SERVICE'

When I experienced this error few years back, I realized 3 things that might be the possible causes for this error:

1. The IIS setting (Virtual Directory) was not correct.
2. The ConnectionString was not correct.
3. Some configurations in the web.config were wrong.

Most of the time, my # 2 and # 3 choices are the common cause of this problem.