When doing:

$(element).add(selector)

the context for the selector defaults to element, rather than the document. While this can easily be fixed by explicitly specifying the context, this behavior can have unexpected and undesired results. For example in an HTML document:

// This effectively results in all elements being added
$('body').add('*');

// Surprisingly, the following calls are equivalent, adding
// only the document and its children
$(document.body).add('*');
$(document.body).find('*').andSelf();