1. Go to Central Administration --> Web Application General Settings under Application Tab
select the web application for which yo wish to turn of this validation error.
in the security validation section select off and click on OK.
This is actually a threat to the site security, This is only recommended for a dev or local site.
2. This is a recommeded way.
Add this tag in the top of you master page.
<%@ Register Tagprefix=”SharePoint” Namespace=”Microsoft.SharePoint.WebControls” Assembly=”Microsoft.SharePoint, Version=11.0.0.0, Culture=neutral,
PublicKeyToken=71e9bce111e9429c” %>
Include the following tag as shown below after the form tag