Continuity mistake: When Horace is confronted by the Gillman and has picked up the shotgun, he looks inside and sees E.J. and another kid hiding behind the counter. Horace begs them to let him in. E.J. and the other kid shake their heads no and lower themselves behind the counter. A few seconds later, a distant shot shows the two shaking their heads and lowering themselves behind the counter again.
Continuity mistake: Phoebe is on the left side of the park bench as she watches Frankenstein fly into the portal. Just before it closes, you see Phoebe move to the very left side behind the bench. When it cuts back to the close-up of her crying, she's back where she originally was.
Continuity mistake: When Wolfman throws one officer toward a car, the officer is shown leading with his left shoulder but in the next shot, which is a close up of impact, he hits with the right shoulder.
Answer: It's unknown how the dog got into the treehouse. After the boys put their hands on top of each others, the dog puts his paw on and Rudy questions how the dog got in with none of the boys being able to give an answer.