When a new release of a web application is deployed to an existing cluster, any changes to the object placed in the web session can cause serialization incompatibilities. To handle this cleanly, we typically recommend that users leverage isolated cluster and mod_cluster load balancing groups (or mod_jk domains) to transition users to the new version of an application.
For simple applications, this might be overkill. It might be acceptable/preferable to create a new session in the event of a serialization incompatibility, rather than failing the request. We should allow developers to configure this behavior per-application.