The following rule being present in the knowledge base causes the StatefulKnowledgeSession to fail to unmarshall with an NPE:

package test.drl
rule "dummy_rule"
when
eval( false )
then
end

Here is the stack trace:
java.lang.NullPointerException
at org.drools.marshalling.impl.InputMarshaller.readLeftTuple(InputMarshaller.java:440)
at org.drools.marshalling.impl.InputMarshaller.readFactHandles(InputMarshaller.java:345)
at org.drools.marshalling.impl.InputMarshaller.readSession(InputMarshaller.java:207)
at org.drools.marshalling.impl.DefaultMarshaller.unmarshall(DefaultMarshaller.java:75)
at org.drools.marshalling.impl.DefaultMarshaller.unmarshall(DefaultMarshaller.java:47)
at test.DroolsSerializationBugTest.testDroolsSerializationBug(DroolsSerializationBugTest.java:87)
at test.DroolsSerializationBugTest.main(DroolsSerializationBugTest.java:28)

Test case attached.
Fails against 5.0.1 and 5.1.0M1