Hello,
I played a lot during BETA for S3 and never had this issue. Since I have started leveling in the battlegrounds with the start of season 3 players are literally invisible some times. I was in a full group of mids and only saw 2 other players while we were together. The SM we were sticking too I could not see but I could see his pet. Several times I would see my group killing things that on my screen were not there. I often see enchanter pets and druid pets running around following nothing.
