I read on one website it was 10%.
Who knows. The USMLE is such a mind **** 🙂 We know so little about it. It's an extremely long test, and it's difficult to actually prepare for in the sense that they can throw anything in the world at you.
Also, with 10%+ of the questions not counting, it's just downright evil, because we spend the 3+ weeks following the test thinking of every question that we saw, agonizing over them, and then they may not even count. Oh man! 🙂