Continuity mistake: After the party, a girl with a maroon shirt and grey skirt runs out of the house twice.
Other mistake: It's reasonable to assume that the house can hide small things like posters and even shelves when projecting images on the walls. When Ben's two friends arrive for the party, one of the walls shows an image and the couch disappears, and that's just way too big of an object to simply phase away, given what we see that the house can do.