In my last game I purchased Regulus's Lvl 15 skill in the wings line (Vengance). It worked fine when I used it, but I noticed later, to my surprise, that I was Lvl 14.

I might have bought the lvl 15 skill with an earlier skill point than that of lvl 14 (I don't remember exacty when I bought it, I was too busy fighting to think it through, hence the eary purchase) It did work, and exactly as advertised, even though I was not supposed to have it yet.

Edit: I did some quick experimenting and this appears to be particular to Regulus's Lvl 15 Angelic Fury skill (Vengance). You can get it as early as level 10. (the place in the Angelic fury line before Vengance) The Vengance skill will open up for selection immediately after buying the Lvl 10 skill. It does not even require a skill point, you can select it for free at lvl 10. (as long as you have the Lvl 10 Angelic Fury skill)