One of the things that `"use strict"` enforces is that you can't make functions within function that are in the strict mode scope. ie, you can't have this:

```js
function foo() {
  "use strict";
  function bar() {
    _.map(someCollection, function(n) { return n * 2; });
  }
}
```

The function passed to map is an error in strict mode. This is probably one of the larger `"use strict"` errors in the page, and the main reason I didn't write it this way to begin with.

[More info](http://whereswalden.com/2011/01/24/new-es5-strict-mode-requirement-function-statements-not-at-top-level-of-a-program-or-function-are-prohibited/)