FederatedPassiveSecurityTokenServiceOperations.ProcessSignInResponse(responseMessage, response);
OR
FederatedPassiveSecurityTokenServiceOperations.ProcessSignOutRequest(requestMessage, User, requestMessage.Reply, Response);
This happens because in the middle of Windows.IdentityModel there's an Response.Redirect("returned page") instead of Response.Redirect("returned page", false) causing the error above mentioned. This is an known issue, and is save to ignore it. So, making use of a catch, you can jump the error and your code would be like...
You can skip this error and hoping for an hotfix soon :)
References:
http://p2p.wrox.com/asp-net-1-1/8403-error-thread-being-aborted.html
http://rmencia.wordpress.com/2010/05/05/federated-signin-requires-federated-signout/
http://social.msdn.microsoft.com/Forums/en/Geneva/thread/b248fafd-f5aa-47f7-9c7a-1ecaa944d2c0
