What PHP fun fact make the use of $last here horribly wrong?

gistfile1.aw

<?php
    $output = '<div id="breadcrumb" class="clearfix"><ul class="breadcrumb">';
    $switch = array('odd' => 'even', 'even' => 'odd');
    $zebra = 'even';
    $last = count($variables['breadcrumb']) - 1;

    foreach ($variables['breadcrumb'] as $key => $item) {
      $zebra = $switch[$zebra];
      $attributes['class'] = array('depth-' . ($key + 1), $zebra);

      if ($key == 0) {
        $attributes['class'][] = 'first';
      }

      if ($key == $last) {
        $attributes['class'][] = 'last';
      }

      $output .= '<li' . drupal_attributes($attributes) . '>' . $item . '</li>';
    }

    $output .= '</ul></div>';