I am seeing this error as well when trying to use the rounder corners with textbox.
I am able to use the rounded corners with the panel control.
I see this error first when trying to run it with the textbox.
htmlfile: Unexpected call to method or property access.
Then it allows the page to break on the line of offending code:
$create(AjaxControlToolkit.RoundedCornersBehavior, {"id":"RoundedCornersExtender2"}, null, null, $get("TextBox1"));
Has anyone else had this error?