Holy Crap

What happened here is that the hacker changed the monsters into different monsters.(ex. flyeyes)
And it'll only change on their screen.
When you or the hacker attack the monsters, the game will DC the attacker, since the monster can't spawn in that map.

BOLD for the win

Last line: I think the game actually d/cs you when you attack a monster that shouldn't be flying, but is. How do I know? A friend of a friend was Dooming the ghosts in Henesys for the event a few weeks ago, but they kept flying around, and every time you attacked one, you'd d/c.

 SSes

Of course, it could also be that Blue Snails shouldn't be spawning in Henesys, but in this case, they actually are Blue Snails to everyone...