Thirty-"we are literally going to have Cassie possessed by a ghost because we aren't even trying, seriously"-Four.
Marco makes a crack about RL Stine in the "possession" scene. But that joke falls flat because that's literally what we are watching: the authors are confronting us with a Goosebumps scene, complete with creepy ritualistic intonations and hokey light shows. It's so utterly shameless in its refusal to respect the context of the rest of the series that it retroactively ruins the Hork-Bajir Chronicles.