jerky.js

function each(ar, cb){
  for(var i = 0, l = ar.length; i < l; i++){
    cb.call(ar[i], i, ar);
  } 
}

function jerkify(obj, methods, thres){
    each(methods, 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);