Being unaware of the performance hit and overusing selectors instead of assigning them to local variables. For example:-

$('#button').click(function() {

    $('#label').method();

    $('#label').method2();

    $('#label').css('background-color', 'red');

});

Rather than:-

$('#button').click(function() {

    var $label = $('#label');

    $label.method();

    $label.method2();

    $label.css('background-color', 'red');

});

Or even better with chaining:-

$('#button').click(function() {

    $(#label).method().method2().css(background-color, red); 

});

I found this the enlightening moment when I realized how the call stacks work.

Edit: incorporated suggestions in comments.