England's Joe Hart aware how costly one World Cup blunder can be

In an instant, with one blunder, a goalkeeper can go from world class to calamitous. England's Joe Hart knows that only too well.

Having established himself as one of the game's most assured and agile stoppers to become his country's No. 1 at the 2012 European Championship, Hart experienced an unexpected setback in his club career.

Dropped by Manchester City last season, paying the price for a sudden bout of shakiness between the posts with high-profile mistakes, Hart's international place suddenly seemed less certain.

"I am well aware that [opportunities] can change really quickly," Hart reflected at the England's team World Cup base.

But Hart is in Brazil with his status unquestioned, a certain starter in Roy Hodgson's team. While City manager Manuel Pellegrini lost his faith in Hart, Hodgson kept his with a goalkeeper who conceded just four times in qualifying for the World Cup.

Although Hart's manner might make him seem too self-satisfied at times, such confidence is vital for a goalkeeper, who knows just how precarious his place in the team can be.

"I've got to be at my best, I've got to be working hard and you've got to stay level-headed," Hart said ahead of England's opening game against Italy on Saturday. "Otherwise it can go wrong."

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City and England teammate James Milner watched at close hand how Hart coped with being dropped. Rather than agitating, Hart fought to win his place back in the team and was stronger following the adversity, helping his side to win the Premier League for the second time in three seasons.

"He just kept his head down at the time, which was the right thing, and it might now be a blessing," Milner said. "Everyone has to go through these sorts of episodes in their careers, and I'm sure he's a lot stronger and a better 'keeper for it.

He's experienced that so, mentally, it'll have helped him for anything else that happens in his career."

Hart emerged as England's top goalkeeper following the team's woeful 2010 World Cup, a tournament defined in the first game by calamitous goalkeeping for Fabio Capello's side against the United States.

Hart watched from the bench as Robert Green allowed a routine shot from Clint Dempsey to slip through his grasp as England was held to a 1-1 draw. Hart still didn't get a chance to play when Green was dropped for David James -- already known as "Calamity James" -- to regain his place in goal.

"Stuff happens," Hart said. "You've got to be aware of that as a 'keeper. Incidents happen all the time. We try to move on and be positive."

Since that tournament, the 27-year-old Hart has done much to help restore the reputation of English goalkeepers, back to an age when Gordon Banks, Ray Clemence and Peter Shilton were admired throughout the world.

Fraser Forster and Ben Foster accept they are playing a supporting role for Hart in Brazil.

"Coming to a World Cup especially, you need to know you have your own role and that there's a pecking order and everyone knows their role in the goalkeeping unit," said Foster of West Bromwich Albion. "We know that Joe's No. 1. Our duty is to get Joe not only mentally but physically prepared for that first game against Italy.

"Whatever we can do in between now and then to get him right, whether that's staying behind to do some shooting drills, we'll do and we will always be there for him."

Forster broke the Scottish Premiership record for minutes without conceding last season but the Celtic goalkeeper but knows Hart deserves his place in goal after being on the losing team just three times in 41 England appearances.

"You can see the performances he has put in for club and country," Forster said. "He's one of the finest in the world."