IMPORTANT - Invisible player complete bug

this player was completely invisible. I logged on, tp'd to her and I couldn't see her. She could open a wooden door and she collected any xp thrown at her. The xp looked like (when it was collected) it was being collected by me from a far distance (the orbs would jump towards VISIBLE players). She reconnected and was visible. this is really important, greifers could harvest this bug and could use it and yes, after tping her she was still invisible. I have the VanishNoPacket plugin but she didn't have perms to use it. (I am 90% sure of that) Someone figure out why this happened! (could it be a VanishNoPacket bug??)