[Fern]

Spawning a monster that walks across the area (with any movement pattern) will often have that monster run up to the person it targets and abruptly disappear, causing no encounter. This tends to make the character it ran into stuck until relog.
  
It's worth noting that when a character relogs a few times after getting stuck that way, they have a decent chance of their save file becoming "corrupted" (as in they become a little soul.) My guess is that it has to do with the constant relogging after getting stuck.