One can observe an interesting trend in Calvin & Hobbes: The Series' Shout Outs.
Given that the story is basically a Continuity Cavalcade based upon the two authors' earlier works, it's no surprise that references to both series come up a lot. But there's a problem - Calvin and Hobbes III: Double Trouble and Trouble Island both had endings that directly contradict the major Cassandra Truth themes on this story! This means that they're probably in Canon Discontinuity. ("The Transmitter Conspiracy" referenced Attack Of The Teacher Creature, mind you.) Or it could simply be that the events of Double Trouble and Trouble Island simply haven't happened yet within the show's continuity.