I cannot provide full evidence of this, however it has happened to me quite a bit. I will try and describe it with as much detail as I can, During a fight I encountered something that was wrong, someone was attacking me but I did not take damage nor was affected by the knockback. Three people within the same level attempted until someone with over 800 levels came and killed me. I'm not sure if this is intended or not, and as I've said It happened randomly so I was unable to record this. EDIT: This happened shortly after a respawn, so perhaps the issue is the respawn protection?
