I have started a second rogue and this one will be leveled with daggers with the aim of being my primary PvP toon.
I have noticed since getting the “Backstab” ability that around 50% of the time I am unable to use it due to an error message appearing reminding me that I need to be behind the target…
Now this is actually happening mostly post Sap/gouge so I am perfectly positioned and have had it happen at a variety of distances from the back of the target.
I do have a rogue alt on Classic now and have noticed that often when I gouge my target it’ll turn to face another way and that sometimes causes backstabs to fail as the game thinks the mob is facing another way than it is on my screen.
I usually get around it by just circling the mob while spamming backstab to find out where the game/server thinks the mob’s back is.
But yes, it seems very, very broken and I’m honestly surprised something this blatantly broken has not been fixed yet.
I made a topic about this on the Class Forum, and this other rogue, Zikpock, explain how it works. I recommend you reading his post in the topic (third to last reply to the topic):
Sometimes however, you actually have to go and face the mob to Backstab. I am baffled that two months into the game’s release we still have to deal with these amateurish bugs.