Login failed after entering valid username & password

This is about a login issue faced by a online user.This user entered the URL of a social networking site & tried to login.But he was not logged in even after entering the valid user name & password.He again tried & this time he logged in successfully.The problem was unique as the application & user details were same.Still user was not allowed to login first time but logged in successfully next time.

The problem was with URL entered by the user first time.First time user entered the URL with www in the start of the URL While developer has set the URL of the login page with out www at the start.This opened the application but the user was not logined after entering the valid username & password.Same time the URL in the address changed to the URL set by the developer & www was removed from the address.This time when the user entered the valid username & password ,he was logined successfully.