Trivia: When Willard's boss is working on his spreadsheet program, just before the rats show up, he is not entering the spreadsheet formula properly. He is entering the actual numbers like 235+345+315 instead of using cell addresses like =A1+B1+C1. Not really a mistake because his character could be an idiot on spreadsheets. Just an interesting observation.
Trivia: The song "Ben" at the end of the movie is being sung by the movie's star, Crispin Glover.
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.