When using PdoSessionStorage, an exception is thrown by PHP when the session is serialized:

PDOException: You cannot serialize or unserialize PDO instances

I think the Session class needs to check if the storage can be serialized, maybe by using a NotSerializable interface, as @fabpot blogged about a while ago.

FYI: this issue is caused by Symfony\Bundle\FrameworkBundle\DataCollector\RequestDataCollector::serialize() and only occurs when I forward a request. I'm using the latest Symfony version to date (2.0.8).