gistfile1.js

var f = function() {};
f.prototype.food = { apples: 0, bananas: 0 };

var f1 = new f();
f1.food.apples++;

var f2 = new f();
console.log(f2.food.apples); // 1

This is because the food object is modified in-place, and both instances refer to the same object.