[edit]
Nevermind, I guess I was mistaken. I suppose the dagger skill "stab" may simply overrule "backstab" sometimes once you get that as well.
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I'm not sure whether this is a bug or not...
The first couple of levels, whenever I stabbed enemies in the back with my rogue (after getting the appropriate skill of course), I am fairly certain it always said "backstab".
But it hasn't said that for quite a while now. Most of the time, it's either nothing or just "stab". At first, I thought it would just be the green slimes (because they don't really have a "back") but now it also happened with an ogre...