Will this for-in loop detection snippet generate unwanted false positives?

We all know that for-in-loops on arrays are absolutely evil. Still, they are often used and the caused errors are complicated to trace down, especially when happening browser-dependent for example because of indexOf-shims or such.

So, I have coded this simple snippet which adds an enumerable getter for a "error" property on Array.prototype (not for use in production code):

Object.defineProperty(Array.prototype, "error", {
    enumerable: true,
    get: function() {
        if (this === Array.prototype) // that looks OK
            return undefined;
        if (window.confirm("Somebody who coded the site you're viewing runs through an Array with a for-in-loop.\nShame on him!\n\nDo you want to raise an Error to trace the origin?"))
            throw new SyntaxError("Array traverse with for-in-loop, touching Array.prototype's 'error' property :-)");
    }
});

You can add it as a greasemonkey script for all domains, and you will see alerts on nearly every site :-) Most of them are caused by calls to jQuery.extend with questionable arguments, btw.

My question is now: Are there any situations that legitimate such "wrong" loops, or anything else causing false-positive alerts?

I am wondering how this would affect the usefulness of my code.