The inside of the house (an actual mansion used; Harefield Grove) does not accurate match the wide exterior shots (which uses a CGI mansion inspired by the real mansion). As a result, rooms in certain locations are inexplicable, if not impossible. Especially many of the bedrooms which are clearly reused for different guests.
After the first night, bodies begin to pile up in the house. They wouldn't take too long to smell horribly and to attract flies and rats and other vermin. Yet there's no sign at all of any of this, even after days.
Near the end, when a character drops the gun (to the accompaniment of a loud crash) the barrel of the gun wiggles, proving that it is rubber.