This has happened to me in Coil. Terribly embarrassing :P

The snake splits into 2, but they both remain next to each other not merging on my screen...when it comes to switching to the alternate snake on the other side of the arena I couldn't do anything - and we would've won if I could've contributed my DPS.

In Turn 2 I often get this with party members being invisible / stuck at entrance when of course they're actually right there with me. Having to run to a specific player to pass Allagan Rot when you can't see anyone else is quite a problem! It's pretty game-breaking and I pray it gets fixed swiftly.