Hi Steve

 I'm not actually getting a Yellow Screen.  Originally I had Custom errors enabled in the web.config and the site was catching the exception.

I changed the CustomErrors setting to Off, and now rather than throwing an exception at the server level, it is throwing a local javascript error.  You get the little yellow triangle in the bottom left and I guess this is because Javascript is doing the Postback.

The full error text from the Javascript dialog is:

Line: 4724
Char: 21
Error: sys.WebForms.PageRequestManagerServerErrorException: Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnabledEventValidation="true" %> in a page.  For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them.  If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.
Code: 0
URL: http://www.myurl.com/VRM/Requests/Default.aspx

Thanks

Steve