The encyclopedia salesmen misidentifies the weapon he is using it is not a Thompson Sub Machine Gun but a IMI Uzi.
At the hotel on the phone with the police, Jason says their home address is 15 Robin Lane, not 15 Robin Hood Lane.
In one of Jason and Maggie's imagined scenarios, the encyclopedia salesman completely annihilates the ringing telephone with an extended barrage of machine-gun fire while laughing maniacally. If he had truly wanted to remain in control by holding the children at gunpoint, he would not have needlessly used up the entire magazine of ammunition just to "shut up the noisy phone", since his emptying his gun would have allowed the children to safely disarm him. Even if he'd had more loaded magazines with him, he would still have been "vulnerable" to disarmament with an emptied magazine, since he would have had to take some time to reload, and thus he could still have been approached and had the gun wrestled away from him while it was still empty.