This is different from #11738 ...

I am generating a large number of forms programmatically using templates (PHP Cake). For each text and textarea there is a select dropdown with a list of tags, and when the dropdown is operated, the onChange event calls a small JS function of my own which appends the tag to the input control's content.

<select onChange="add_tag('caption_3',this);" ... >

The function uses JQuery to locate the adjacent input control and do the append, like this:

function add_tag(target, select) {

   // jQuery bug
   var form = $(select).closest('FORM');
   if ($.isArray(form)) {
      form = form[0];
      console.log(target + ' got an array');
   }
   
   elem = $(form).find('[name=' + target + ']')[0];
   if (elem.nodeName == 'TEXTAREA')
      elem.innerHTML += select.value;
   else
      elem.value += select.value;
}

As the code implies, I sometimes but rarely get an array returned by closest() rather than a direct reference to the element. I have been unable to figure out exactly what circumstances trigger this problem.