Maybe your browser is too old to handle the script, maybe the script author made a mistake, maybe you entered some data that triggered an error that the author failed to handle, e.g. entered zero for a weight, or maybe the network connection got interrupted at the wrong point and some data only got half loaded.

Are you using Windows XP and Internet Explorer 8? If you do, switch to a browser that isn't outdated by 5 years. Firefox or Chrome.

if thats when you have a browser open i would say its the browser