It's when you spawn a walking monster. If that monster runs up to someone, they will frequently make that character get stuck on contact (as in they can't move until they relog) and the monster will disappear causing no encounter.
If you want to try it out on yourself, just spawn a walking monster and let it run into you until the time you get stuck.
If you want to try it out on yourself, just spawn a walking monster and let it run into you until the time you get stuck.