Plot hole: In this film, the Yeti is Sulley and Mike's co-worker in the Monsters, Inc. Mailroom. But in Monsters, Inc, when Sulley and Mike are banished, they apparently don't recognise him.
Plot hole: During the final scare event, Mike gets a perfect score because Sully broke the simulator to allow the 6th scarer to get a perfect score no matter what. After the contest, the simulation resets and Mike pretends to scare the child in the simulator and it registers a perfect score on the scoreboard for the 6th scarer when it should really have registered a score for the 1st scarer, because the simulator had been reset.
Answer: Well a door first needs to be activated from the monster side. See the doors of the human world lack the receiver for the energy they need to open the doorway between worlds (the red light on top). That's why in the monster world they hook a door up to a machine to activate it allowing the passage between worlds before any scream or laugh is made. It costs power to activate a door to allow a monster in. Boo was able to activate doors because she was in the monster world and her screams and laughs triggered the devices on top of the doors, activating them automatically. This was the first time ever a human entered the monster world.
lionhead