I've tracked down the problem to the following lines of code within the effect.core.js script

The following lines within the createWrapper function will produce under certain circumstances NaN values for top & left.

var top = parseInt(el.css('top'), 10); if (top.constructor != Number) top = 'auto';
var left = parseInt(el.css('left'), 10); if (left.constructor != Number) left = 'auto';

fix:

var top = parseInt(el.css('top'), 10); if (isNaN(top)) top = 'auto';
var left = parseInt(el.css('left'), 10); if (isNaN(left)) left = 'auto';