chaos monkey. If your tests pass with this in page your tests are wrong. or you are very very lucky. buy a lotto ticket.

jerk.js

function jerkify(obj, methods, thres){
    methods.forEach(function(meth){
        var orig = obj.prototype[meth];
        orig && obj.prototype[meth] = function(){
            return Math.random() > thres ? orig.apply(this, arguments) : orig.call(this);
        }
    });
}

jerkify(Array, ["push", "pop", "splice", "slice"], 0.1);