This happened to me during a fate today. A mob was using an in-line AOE and I got my back against it cause I was running to turn in my items and I parried it. I was honestly surprised myself.
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Area effects that are physical can be blocked from and facing (Liquid Hell, Fireball from Twintania). Auto Swings/Single target Skill can only be blocked at an angle about 60 degree left and right with a total angle of 120 degree.
However when you attack a mob you automatically face (this is set as default unless you manually changed it). So in that sense you technically are "facing" a mob. An interesting thing that I noticed is when I spin the mobs in BCT 4. I am not locking on and I know for a fact that at times the enemy is perpendicular or greater in angle, yet I still can parry. Yes it is auto attacks and yes we all know spinning them reduces their attack rate but this is what I see. Then again with so much going on with proper pulls and top tier aggro management I could just be seeing things. I will have to test this out on low lvl mobs as the parry rate is much higher on them w/o using any buffs.
i have dodged some special attacks from behind as i was running past or through a zone and had agro. I haven't seen parry though, I'll test this more when the server comes back up later tonight.