We use mode_cluster together with a cluster of Tomcat containers. Each container might have multiple versions of the same app deployed (using the parallel deloyment of Tomcat together with automatic undeployment of old versions). Now consider the usecase:

new version of existing app is deployed to Tomcat
old app loses all the active sessions
the old app is undeployed
mod_cluster removes the corresponding context even though there is still one existing (the new version)