System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

In web.config write following...
"page enableviewstatemac="False"

The name 'Request' does not exist in the current context

This error normally occurs when you are trying to access a property such as Request.Cookies and you're not doing that from a webform, usercontrol or a class that inherits from System.Web.UI.You can still access these properties, but using a different path.
For example to access a cookie with the name MyCookie you would normally use Request.Cookies["MyCookie"]; however, if the error The name 'Request' does not exist in the current context is returned, use the following path for retrieving the cookie:
System.Web.HttpContext.Current.Request.Cookies["MyCookie"];
This eror may also occour for "Server" etc..