Strange error that occurs randomly...

HI,

I'm using flash builder and I have a strange error, this error occurs randomly and in order to fix it I do the following:

1) Remove the line where the error is

2) Save the file (so the workspace is rebuild), the problem is gone

3) Paste the line removed

4) Save again and there is no problem

Here's a piece of code:

protected function comboHandler(event:ComboEvent):void
{          u.profilo = Profilo(event.picked);
}

The ComboEvent picked is of type Object

The u is referring to a class where there is a getter and setter

public function get profilo():Profilo
{          return profilo_value;
} 
public function set profilo(i:Profilo):void
{          profilo_value = i;
}

The error I get from Flash Builder is

1118: Implicit coercion of a value with static type listClass:Profilo to a possibly unrelated type listClass:Profilo.

Why do I get this error?

Is it related to flash builder?