Other mistake: During the cut scene where you see Abaddon before fighting Straga on The Apocalypse/Mayhem level at the beginning of the game, you can see Minion Demons freely crawling out and diving into the pool of lava unharmed and dragging Angels into the lava. However, during game play on other levels if War hits a Minion Demon and knocks them into a pool of lava, they die instantly.
Trivia: The Watcher is voiced by none other than Luke Skywalker and the Joker himself... Mark Hamill.