I’ve had this happen with Tuna, sometimes Krait and undead in Caledon, the latter two even on land. I’ve observed that sometimes they have intersected with an object in surrounding environment, much like MOB in other games that get stuck in objects and become un-targetable or invulnerable. Other times, the lag means I’m casting on them at one location where the game server and the client on my computer are slightly out of sync. That’s pretty common on crowded servers.