From Thinkgeek:
Put out this Critical Hit D20 Doormat to forewarn visitors that if they bust open the door they might experience a Random Encounter. In a high-rise or a dorm, it’s a great way to mark your territory if you play pick-up games with folks who might not have visited before. And, of course, it’s a nice way for your abode to welcome you home. Instead of reading “welcome,” your doormat has rolled a critical success. How YOU doin’, high roller?