Here's a bit more info...
The code for Widget is something like this:
" type="text/css" rel="stylesheet">
I'm also using the following version of Telerik.Web.UI:
Debugging the issue shows that the error occurs at the following syntax:
This would appear to be Telerik code, as it doesn't seem to be in the included js file shown in the above Twitter code.
I'd put in a couple of "alert" calls either side of the Widget object creation code, to try and see when the error occurs, and the error seems to occur some time after the object is created (I'm guessing now, but maybe on callback after a json call?)