Simple reproduce case:

  <html>
  <head>
  <script src="http://code.jquery.com/jquery-1.6.min.js"></script>
  </head>
  <body>
  <script>
  $('body').append(
    '<span></span>',
    '<span></span>',
    '<span></span>'
  )
  </script>
  </body>
  </html>

The bug is in this block: https://github.com/jquery/jquery/blob/1.6/src/manipulation.j...

Note the inner loop is clobbering the outer loop's "i" variable.