This exception is thrown by PDO because PDO instances are not serializable for good reasons. But it is weird because the sfForm class does not depend on PDO. How is it possible?
After some poking around, he noticed that this was only an issue for those with sessions stored in the database (that's where the PDO comes in). The problem came with the validation error class - it extends the exception class and, because of how the error handling serializes the exception, it tries to serialize the PDO connection that's in the stack trace too.
His solution is to use a Serializable interface to define exactly what you want serialized and passed back out as the error to the rest of your script.