Sometimes while in a party, if a member is riding or dismounting a chocobo, they will appear to be stuck, hovering over an invisible chocobo. When this happens, you aren't able to target or use any actions on the player, but on the players screen everything is fine. Some members in the PT can see them, while others see them floating frozen.

This also sometimes happens with the fly monster in Skirmish after it does it's teleport move, when it reappears, it will still be invisible and untargetable for a few seconds, but still able to attack the party.