[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.

[Neus]

Needs more details on when this occurs and with what.

[Fern]

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.

[Neus]

You'll have to try this on the testing server and see if you can get a way to consistently reproduce it.

[Fern]

It happens in the test server as well. I've gone ahead and recorded the bug in action. It has made me get stuck consistently with the following encounter:
makemegetstuckgroup.sav
 
Video recording below. If you want to try it on yourself in the test server, just use the encounter I mentioned.

 
It also consistently makes other players get stuck. I've actually not seen it properly start the encounter in the test server at all - it's always making someone get stuck instead.
 
Note: The video recording doesn't show the windows that pop-up when you're selecting encounters and stuff for some reason...

[InsainArcaneBirdbrain]

To add on to this, it doesn't matter if the mob spots the player and runs into them, or if the player back attacks. Any instance of touching the eventmin spawned mobs on map will freeze player up and stop them from moving/leaving party/etc akin to how they freeze when the screen is fading to black into battle.

[Neus]

I doubt this was related to any sort of save corruption, but it has been corrected anyway.