If you have the xCDN module and it is active it is possible the "estimate shipping" popup on cart page to not load properly causing the site to either crash or behave strange. To fix this issue follow these simple steps:
{include file="check_zipcode_js.tpl"}
{include file="change_states_js.tpl"}
{*include file="check_zipcode_js.tpl"*}
{*include file="change_states_js.tpl"*}
NOTE: Make sure to clear the cart cache for the changes to take effect. You can do this from admin - Tools->Maintenance menu link.