This week I discovered a new bug - occasionally, players and mobs become invisible in combat. I'm not sure exactly how and why it happens, but once in a Chaosball battleground an enemy player picked up the ball and became invisible for three out of four group members on our team. I believe it might have been the players that died and had to respawn all together, but I am not completely sure on that.
Another time I loaded into one of the Imperial City districts and ran towards the flag, capturing it, and an invisible Xyvkin Nightblade kept attacking me, so I had to spam aoe attacks, and it got revealed only after its death.