While I was working for one of my customers, we ran into a very strange problem when they tried to deploy some WCF services in a web farm using cookie sessions (In order to enable secure conversation in this kind of scenario, cookies must be used to track the state of the Secure Conversation Tokens).
At some point, we start receiving a random exception with the following message,
System.ServiceModel.Security.MessageSecurityException: Message security verification failed. ---> System.Xml.XmlException: There was an error deserializing the security token XML. Please see the inner exception for more details. ---> System.ArgumentException: The SecurityContextSecurityToken with context-id=urn:uuid:502e208e-8db2-4814-8623-d3050273e875 (no key generation-id) has expired
Later, we found out that a time skew setting was not supported in WCF for the SCT cookies, and the error was happening because of an slight difference in time between the servers in the farm.
Fortunately, Brent Schmaltz has made public a hot fix for that problem in his blog. You can find the complete solution and a better description of the problem in this post.