[YogiTW]

The problem is vB's use of the for/in loop: for (var i in array_var) { }

When you alter the array prototype, those new values show up as i. For example:

Array.prototype.wtf = function() { }

var s = new Array("1", "2", "3");

for (var i in s) {
  alert(i);
}

you will get pop ups that say 1, 2, 3, and wtf.