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.

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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.