Factual error: After the intruders flood the panic room with propane, Jody Foster's character gets a lighter and ignites the propane causing it to burn along the ceiling. This would be impossible as propane is heavier than air and would sink to the floor rather than rise up to the ceiling. Lighting a flame in that room should have caused anyone in the room and on the floor to be engulfed in flames almost instantly.
Revealing mistake: Near the end of the movie Dwight Yoakam is holding Jodie Foster's daughter captive as he tries to exit the house. Jodie Foster sneaks up behind him with a sledgehammer and swings it at his head. He turns to face her and the metal end of the hammer catches him firmly in his temple, knocking him down a flight of stairs. A few minutes later he comes up the stairs and tackles Jodie Foster to the ground seemingly unhurt by the impact of the sledgehammer.
Question: I don't really understand why Jodie Foster's character didn't tell the police something was wrong when they came to the door. I know she was worried for her daughter but the robbers couldn't overhear and she could have asked the police to come back later, giving her a chance to smash the cameras. Also, considering that she emphatically denied there was anything wrong, why did the police return later with all the guns etc?