When entering a fight with a monster, especially aggressive monsters, it’s possible to become ambushed and unable to walk out of the room for up to 3 rounds. This is a particular problem for newbie explorers, who don’t know where an aggressive monster might be, and have the least ability to survive.
There are ways to avoid or resist being fully ambushed. Having high dexterity, intelligence, alertness skill, and being small sized are all things which can help avoid ambush.