Ok I don't know if this has happened to someone else. I was fighting in arena in the map where there are pits with spikes all around the borders. I realised that if you throw yourself in the pit right after you won a round, the next round you appear invisible, so your rival can't see your health or what are you doing (blocking, kicking...). A guy did that to me and it's incredibly annoying -.- Sorry I don't have screenshots, but the next time it happens I'll post them.
Customer support service by UserEcho