Suppose you open a Modal Dialog window inside your ASP.NET page. In that dialog window, you have a GridView, DataGrid, or whatever control. Do some changes on that control, you will see that the data has changed right?
Ok, then close the dialog window, then open it again from the ASP.NET Page, you will see the old data and not the data you changed last time, right?
Ok, this is because the data has been cached by the Modal Dialog. It seems that this is how Modal Dialog Windows are built to cache.
To overcome this, you would need to add the following in the top of the ASP.NET page that is being opened with the Modal Dialog:
<%@ OutputCache Location="None" VaryByParam="None" %>
Hope that helps,