Half-Life 2's Invisible Tutorial | Game Maker's Toolkit

"The first spinning blade machine you come across in Ravenholm forces you to crouch beneath it. When you are halfway from crossing it, zombies pop out of nowhere and begin to approach you. Naturally, the player will retreat because it is difficult to fight in this position. By the time that you're in the clear again, the zombies have walked into the blade machine. The lesson: "Zombies will walk into the blade machines and kill themselves." The next room has another blade machine, but it's off." - from Doublicon