Example: http://live.datatables.net/yupeqan/1/edit
If you use deferRender and then pass a string to the rows() function, you will get the following error in the console:
Uncaught TypeError: Cannot read property 'nodeType' of null
Looking through it with a debugger, this is because the elements array passed to jQuery.grep has length equal to the total number of items in the table, not just the rendered ones. The rendered ones are their respective nodes, and the rest are undefined which throws an error when accessing elem.nodeType.