Compiler Warnings 

function f(){
   this.b = 22;
   this.a = new Array(2);Â  
   this.a[0] = 33;
   this.a[1] = 44;
   return a; 
   } 
   // returns a new instance of f in ActionScript 2.0 and a new 2 element array in ActionScript 3.0
   var d = new f();Â  // Warning here
   trace(d.a);       // undefined in ActionScript 3.0, [33,44] in ActionScript 2.0.

class a 
{ 
   var x; 
   function a() { x = 1; } 
   function b() { trace(x); } 
}

var A:a = new a();
var f:Function = a.b; // warning triggered here
var x = 22;
f(); // prints 1 in ActionScript 3.0, 22 in ActionScript 2.0

trace(NaN == NaN); // false!
trace(NaN != NaN); // false again!
trace(isNaN(NaN)); // true

var x:Boolean = false;
var y:Boolean = true;
// it is hard to determine if the line below intentionally sets x's value to y's or if its a typo
if (x = y) { trace("x is assigned y's value of true, making the conditional test evaluate as true."); }

 if (x == y);
{
	trace("This code will be executed no matter what are the x and y values.")
}

function test() {
	var x:Number = 10;
	if (true) {
	    for (var x=0; x < 5; x++)  // warning here, this is the second defintion of x
	    trace(x);
	}
	trace(x); // 5, not 10.  The last value set by the for loop above is the current value of x
}