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A player managed to find that his pokemon randomly go inside the PC for no reason, even when he hasn't touched a PC recently. Another player I found was battling with a venusaur got stuck in battle, did /pokesee and he didn't have venusaur on his team but when I did /compsee it was there in his PC. This may be one of the many causes for being stuck in battle.
In most cases though, the player experiencing this bug would be healing their pokemon, to find out that one of the pokeballs didn't show up in the healer - the bugged pokemon so they would try to place them in the PC and out again - which worsens this since it dupes it most of the time.
Again these are just observations. I still do not have actual steps to reproduce this, since the nature of this bug seems to be random (at least from this standpoint)