I'm looking at the following code.

    _create: function(html, doc, tag) {

        tag = tag || 'div';


var frag = Y_DOM._fragClones[tag];

        if (frag) {

            frag = frag.cloneNode(false);

        } else {

            frag = Y_DOM._fragClones[tag] = doc.createElement(tag);

        }

        frag.innerHTML = html;

        return frag;

    },

There are not motivational comments included but it appears that the programmer wanted to avoid creating elements if an element with the same tag was already created.

The problem is that this function takes a "doc" so that when the element is created it is created with the correct doc. If a subsequent call to _create is made with the same tag but a different doc,
the element from the old doc is used which is likely not correct.

Y.DOM._create('alpha', doc1, 'p');

Y.DOM._create('alpha', doc2 'p');

In order for the caching to work so that the right frag element created with the correct doc object, the caching needs to be done in two dimensions (i.e. tag and doc) but it is only being done in one
dimension now (i.e. tag).