C# UpdatePanel Update seems to be working in Firefox but not IE

I've got a page with several Update Panels on it. The first has a panel with a gridview which has an event to close the panel and update the UpdatePanel - then populate a control in a second updatepanel. It seems to work fine in firefox but in IE the panel appears to not respect the visible=false. The gridview is not databound so it doesn't show but there is still a space. Firefox works as I would expect.

In IE the 2nd update panel which shows has a few buttons, etc in it. Clicking on any of these throws this error.

Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: Invalid postback or callback argument. Event validation is enabled using in configuration or <%@ Page EnableEventValidation="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 t开发者_StackOverflow社区hem. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

During the page load the code was changing the value of a single label that was not part of the update panel but it was being manipulated after the update. Simple little label :-/

I ended up stepping back version by version through subversion until I found the break and inspected the updated files one by one - found the place where we added that little label.