Other mistake: In the scene where Willard is in his bedroom closet, he is trying to block the hole that Ben has created. On a closet shelf there is a Christmas Decorations box. The exact same box can be seen on a shelf behind his boss in the office supply room. (01:01:55 - 01:09:30)
Other mistake: Real supermarkets do not put cornflakes and rat poison in the same aisle. (00:04:35)
Answer: Pure marketing. The original ending was the shot of Ben in the window, but test audiences thought the movie was too dark already. The rest of the end was filmed months later, with Willard stabbing a bean bag.