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If we get the above error in SharePoint, you need to add one tag in the aspx page,
 using Microsoft.SharePoint.Utilities;
<SharePoint:FormDigest ID="FormDigest1" runat="server" />

protected override void OnInit(EventArgs e)
        {
            if (Page.IsPostBack)
                SPUtility.ValidateFormDigest();
            base.OnInit(e);
        }